Utility and food expenses should be split in thirds - you are right, there is essentially another adult now living in the house. If your DH needs proof, which it sounds like he does, start showing him the bills and compare them to before SS moved in. Many utility companies now have fixed expenses that are the same every month in addition to the actual usage expense. It is easy to look at your bill at find them. You could split the fixed expense in half and the usage expense by thirds.

Is this type of attitude typical for your DH? He seems pretty dismissive of your valid concerns.

There are 3 people living at your home. For shared living expenses, I would recommend you pay 1/3 and your DH pay 2/3. If you had your 2 kids (not w DH), would he still want to split bills 50/50?  Doubt it. 

Utilities -- I'd look at the bills before SS moved in and after. Prices have changed so don't look at cost. Look at usage to find a fair percentage. Compare month to month -- January to January, not January to July.

School tax, property tax, internet, mortgage-- SS being there does not affect these bills so these stay 50/50

Food -- this one is tough. Most 15 year old boys are bottomless pits. 
Basic food, staples -- I'd probably stay 50/50. Yes, you'll pay a little more, but (to me) it's not worth counting corn flakes. But, the snack, special drinks, and special items SS wants should be his father's responsibility. 

You were not wrong to bring this up, and your H's reaction to your reasonable request is ridiculous and a bit childish.  Is your H giving you half the child support he receives for SS?  He doesn't get to have it both ways.  He can't keep the revenue SS generates but not expect to pay SS's costs.

I wouldn't drop it.  Do you have the bills that show that your costs have gone up since SS arrived?  Your H should be paying the increase.  I would let him know that you're not trying to nickel and dime him (kind of his approach with the falling asleep in front of the tv and running a fan), but that SS's presence in your home should not increase your expenses.  How does the amount of CS your H receives compare to a third of the household bills you're splitting?  If it's less, perhaps a compromise could be that the CS for SS goes to household bills and you split them after that.

I think you were right to bring it up.

To my mind, SS is nothing to do with you so why should you pay half of his costs which is essentially what's happening here.

Your H can use the CS to put towards his sons costs, then pay the rest himself.

He sounds a bit mean....

Generally, the cost of the home would be split 3 ways.. I am assuming his son has his own room right?  Now.. many couples will prorate the cost of the home, utilities, groceries etc.. on a prorated basis that takes into account the person earning more.. paying relatively more.. but in that scenario.. yes.. HIS CS would be added to his takehome to make that determination.. in that light.. perhaps he is pretty much even... or close enough to not worry about that proration.

I think he is being stingy and he absolutely is ignoring the fact that his son is using resources in the home.. that YOU are paying for.. and he is NOT your son.  

If your DH were to live alone with his son.. he would likely be renting or buying at least a 2 bedroom home right?  if you were to live alone.. you could get by with ONE.  CS is meant to cover the cost of things like housing.. transportation.. utilities.. food.. clothing etc.. so he is reaping the benefit while you carry some of the burden.. and that is unfair to YOU.

You may have not seen it as such a big deal when he was not there FT.. but now he is.. he is using the resources.. and I don't know why your DH feels YOU should foot the bill for his son.  It's not jsut that costs have gone up.. it's that he is expecting you to cover half of his responsibility.. and that is not the same thing as pointing out that he took a long shower.. he always should have been paying more.. and it could have been relatively "less" more if the boy wasn't there FT.. but he is now.. and that is the basis for the 1/3 split.

I am not addressing issues like who's name the home is in.. that can sometimes impact the split too.

You are not wrong to bring it up, SS being there full-time does make a difference and you shouldn't have to pay for that difference.

Your husband is being unreasonable and selfish.

The thought that HE shouild be financially responsible for HIS kid??? What's wrong with you for asking him such a thing! Of course, I'm being facetious with that statement. And YES, you have every right to request he pay more. It has zero to do with you falling asleep watching tv or using a fan. Why should YOU cut back on utility use just because he's being held responsible for HIS kids' utility usage? Gas Lighting. That's what you got with his behavior. 

I'd definitely bring this back up and I'd tell him up front to quit the BS behavior. Is he expecting you to cover HIS kid's college expense, too? Have him hole up at your house till he's 40?? You need an intervention on this issue immediately; probably need a counselor. Best to you.

Because SS is living, eating, using electric,water,   He has to understand that. He just being a jerk 

That's the bigger problem here.  ANY reasonable person of average intelligence would realize that adding a third person to a household means greater expense. 

If you and DH added any other "roommate" to your home who was using resources, you would certainly expect to have them pay a third, right?  So why not his semi-adult kid?  Especially since he is getting child support for the kid - which is supposed to be used for exactly that purpose.

My bigger issue is the way your DH responded to your inquiry.  To be sarcastic and hostile when you wanted to discuss something entirely reasonable is a big flag to me.  He has mentally/emotionally browbeaten you into not confronting him or challenging HIS status quo when it comes to money.

SS is going to be around awhile so this issue needs to be resolved.  My concerns, based on how you describe your DH's reaction, is that even if he agrees to a greater amount of money he will do so with great reluctance and will continue to criticize and marginalize you.  I can see him making snide comments every time you take a second helping of food, or do an extra load of your clothes, or buy a food item that only you eat, etc. 

I will give my standard advice:  If worse comes to worse, please ensure you have a financially viable exit plan if you need one, so that you can move out and support yourself.   

Be blunt. Having a third person living with you increases ALL the bills (except for rent/mortgage) by 50%. Does he seriously think you are going to cover that? Time to sit down with the bills and work them out. Oh, and kiddo will only get the food that the rest of you are eating, if he wants extras that's up to your SO to cover.

Please tell me your finances are separate ...

Nope, he's being difficult on purpose so you'll drop it.  In our house we have a joint account.  So this is not an issue.  But it sounds like money is a touchy subject to the two of you.  If it wasn't you woudn't be splitting bills in the first place.  You'd just be putting it all in one pot and sharing the highs and lows together. So he knew up front that this is an area where you have strong opinions.  

If you were to get a roommate (say your sister for instance, so it's family) charge them rent, expose your DH to the hassle of having someone else in their space all the time, keep all the rent money yourself, and then expect him to continue paying 50/50 of all the expenses he'd object pretty quickly to the increased electric, water and food bills.  What he's doing is no different just because it's his son.

We had/have only a joint checking account, an overflow savings account and our income (regardless of who earns it) is marital income.  We had SS from age 2yo to 18.  DW was the CP and received CS (mostly a pittance until the last 5yrs) which went into the family coffers.

We both have 401K and IRAs though we are each the others beneficiary on those accounts.

Since we were an instant blended family and DW and I raised SS together from age 2, I never had an issue with splitting the bills based on the biology of a child resident in the home.  As it turned out, SS-30 is an only child in our marriage. DW was an SAHM the first 3 years of our marriage then went to work. I never took issue with supporting SS.  

I might have a different  philosophy if he had been a deadbeat adult Skid or been a waste of skin as a teen.

I won the blended family lottery.  I am very fortunate.

OP, why did ss move in? Did you have a voice in the decision? Why only him and not his sister? 

The cost of housing a kid is the incremental cost of having the extra bedroom.  So if a 1 bedroom costs 1500, and a 2 bedroom costs 2000, housing the kid costs 500.

Utilities can be split three ways.  If your SO wants to be a butthole, I'd say your electricity usage waste is more than offset by SS's masterbat!on showers.

Food.  I'd say that two grown men eat way more than you do, most likely.  Splitting that three ways would be very generous of you.

The two adults split the housing costs based on income. If the household income is $100,000 and you earn 60k of that and your husband earns 40k (including child support), you split the housing bill (mortgage/escrow or rent) 60/40. The reason being that if you weren't together, each adult would be responsible for paying to live somewhere AND as a couple, no one is trying gouge the other. It's only fair that the higher wage earner contributes a bit more.

All fixed and variable costs are split by the number of consumers in the home: Internet, electricity, water, food, etc.

From here on out, DH pays for groceries and utilities. You'll give him your third of the cost. If he's resistent just stop grocery shopping and change the utilities into his name if they aren't already.

He should not just be paying more for food and utilities but also the room/s your stepson is using while he's there. As long as he is there he has sole use of them so should be paying for them (calculate what the rent would be per room plus possible bathroom and your husband should cover it).

I posted above because you made it clear that you two have already split finances.  But just for the added perspective - My DH and I do the same as Rags.  All the money goes into one pot.  I've never understood dividing expenses by who makes the most.  Sometimes I did, sometimes DH did.  But we both put in the sweat of our brow or the angst of the office 40 hours a week.  Just because one of us was paid more didn't mean the other was valued less inside our own home.  Our children's expenses all came out the family income.  I think when you start keeping score, inevitably someone loses. 

So this wouldn't be a hill for me to die on.  Yes expenses have gone up.  Yes for a few years you might have less discretionary spending.  But would you really want to start keeping money to yourself for treats and outings that your DH now can't afford?  I get it might seem "fair", but is that the relationship you want?  If both of you are hard workers, and both of you are devoted spouses, perhaps you want to take a less rule-oriented view on finances and a more relational one?

Who paying for SS school clothes, trips, school expenses, spending money . ?

THEN 

When SS needs a Car. Graduation party, college expenses. Wedding ect. Who going to pay for these things. BM not

I recommend you pay 1/3 and he pays 2/3 since he has two kids living there. Do you ever feel like you are being taken advantage of?