He is making a choice.  I would be telling him unless he stops this, ramps up hygienie, he starts losing his priviledges.  

For this. Buy a large plastic kiddie pool like we did training puppies. Put a sleeping bag in it. SS or DH can wash the sleeping bag at the laundromat. Might have to take other things out of his room to allow room for the pool...like his TV or gaming system. And babies that wet the bed go to bed early and they certainly don't have company over.

Holding /stopping drinks at 630pm will not make him dehydrate. IF your concerned about that, call his doc and ask.

Many parents stop drinks early evening as part of potty training. It does help. 

A few sessions with a Phd in child psychology may help more than anyone thought.

Why isn't his father washing his sheets and cleaning his bed? How is this in any way your responsibility???  

If the stench is horrible get a large piece of thick plastic like they use for painting and tape it around his door.  Put his clothes in a garbage bag and tie it shut, leave it for dad or send it back with him to BM.  

Dad, mum, and this kid all have issues.  

...he doesn't wet the bed all the time, sometimes, he won't wet at all on his visitation weekends. Other times, he will wet both nights. He now knows how to turn on the washer, but sometimes leaves wet blankets in his room. Also, it's annoying when I trot into the laundry room on Saturday morning to put a load of clothes and my washer is occupied with pee-stained sheets. 

DH doesn't say much to him because he thinks he'll be embarrassed. BM supposedly has pills that will keep him from wetting, but she would never send some to our house. I'm supposed to pretend I don't notice, because SS would be embarrassed. At this point, he should be embarrassed. He's 12. I don't think BM or DH has ever talked to his doctor or a specialist about his issue. 

There was someone on here whose skid was wetting the bed still at 15. Just laying in his own urine. That seemed related to having a crazy BM, but ...

How often do you have him? The best way to deal with bedwetting is to put plastic covers over the bed, make him get up and change his sheets and wash the other bedding, without anger if possible (on DH's part), and offer him rewards for every night that's dry.  I know that seems crazy for an 11-year-old, who should have been dry for 6 years now, but yelling at bedwetting kids just makes it worse.  His reward should be something he earns after several dry nights - ie, if you have 10 dry nights you can have a sleepover (or whatever appeals to him).

BM is emotionally abusing him by not making him learn to sleep through the night without urinating. Also, call his doctor and get some of the nasal spray for your house. It works.

If he doesn't wet when he has overnights, he can control it. This is your DH's problem not yours. You are enabling DH and SS to ignore the issue. DH should wake SS every 2 hours to make him use the bathroom. If there is an accident. SS needs to wash and dry all linens. Your DH is responsible to check behind SS. If SS was embarrassed it wouldn't happen. 

Put a waterproof cover on the mattress. It will at least save the mattress. What about putting him in older kid diapers? A little shame might go a long way to curing this if it's not something bigger.

Definitely stop fluids early in the evening. It might not stop it, but the mess will be less.

I always thought bedwetting in kids was associated with developmental/mental health issues, but I don't have any kids so what do I know? My dog literally waits at the door to come inside and pee. Thank god I didn't have kids. If my dog training abilities are any indication, my kids would have been in diapers in college.

Has this been brought to the attention of SS's pediatrician?  Has he been examined by a urologist?

My SO's daughter is 8 and we had issues with her wetting the bed nightly.

I told my SO how I 'trained' my kids to use the bathroom at night by setting an alarm and waking them up until they no longer needed me to do that. My SO fought me on this so hard. What it boiled down to is that he was lazy and did not want to do this. He made excuses that were total nonsense like, saying it's unhealthy to wake children in the middle of the night.  I fought with him on this until he agreed to try it my way just to appease me. I'm pleased to report that was months ago and such a success that it continues now. 

So, when she is at our home, my SO sets an alarm to wake her in the middle of the night to use the bathroom. And SO is always in charge of washing her clothes and linens. This also puts 100% of the responsibility on SO. 

This has almost eliminated the night time bed wetting during the 4 days a month that she is at our home. (She only had one accident in several months, and I honestly think my SO failed to set his alarm that night). It hasn't taught her how to use the toilet at her mother's and she continues to just lay in piss there. But at least at our home, she doesn't anymore. 

I had a problem with my sgkids for years. My sgkids had a problem wetting the beds and sd played her little games of "oh, sgkid had an accident". My beds were wet everytime they came for a visit and every night they were at my home. I don't blame the kids as I really feel they can not help it. I felt sd used the kids problem as a way to get at me and I really got tired of washing bedding every single morning just to repeat it all again the next morning. My dh was no help at all and just totally oblivious of all my exhausting work taking care of the beds.

Here is my advise.1)  Get mattress covers for all your bed. There are nice ones that are even comfortable. 2) Buy pull-ups and insist ss wear one everynight regardless if he has friends over or not.

I had some big fights with my dh about these wet beds. I had finally gotten to the point that my beds were going to be dry. I didn't care if sd ended up going to a motel with her bed wetting brood or if they slept out in her car. I was to the point that divorce looked better than putting up with deliberately wet beds on the part of my sd. She was fully aware that her children had bed wetting problems and still do. After me asking every single time they came if she brought pull-ups, she finally started bringing them. I provided the pull-ups until sd finally arrived at the realization that I was going to ask her every single time they came for a visit. It is HER responsibility to provide diapers, pull-ups etc.

Good luck

This is a control thing IMHO. It gives him control and it manipulates the adults in the picture.

The solution is not what he wears at night but what he is forced to wear during the day.  Tell  him one more incident and he goes in a diaper all day every day including to school.  Not one that will be easy to hide.  A big fluffy diaper that his peers will have a whale of a good time ridiculing him about.  Take all of his clothes other than a small selection that will make the diaper clearly identifiable to his classmates.

Take all of his electronics. If the are distracting him from getting his lazy disgusting ass out of bed to take a leak then they are gone. 

A life of abject misery and public humiliation works wonders on this kind of crap.

Throw in a bare butt paddle to ass session when he does this crap and you have all of the abject misery bases covered.

Lather, rinse, repeat.

My DS11 also wets the bed about 3-4 times a week. He's mortified by it. I get frustrated because I can't wash other laundry due to washing his sheet/blanket every day.  We do have a plastic cover on his bed, and I clean it with antibacterial cleaner. I don't believe he does it maliciously or intentionally, and his pediatrician thinks he will outgrow it. It sounds like you're describing your SS somehow is so lazy he's fine with wetting himself rather than going to the restroom. Perhaps if I were a SM to my DS instead of a BM, I'd think that. ha ha ha. If that is true of your SS, and you believe he can control it and just chooses to urinate in his pants and on his bed, it sounds like he needs to see a psychiatrist to address it. 

3 things:

Plastic sheets.

Also, this kid is 11, he should 100% be washing his own sheets! Even if he has to be told to do it, since he apparently has no issues with laying in his own filth.

Consequences.

Right now there's no real reason for him to stop. No motivation. Everyone just cleans up after him and he doesn't mind it, but I bet he will mind washing his sheets and losing privileges.