What happened to his plans to join the army?

IMHO.. your DH should let his Ex make her own decisions on what she asks him to contribute.. your DH should not contribute anything.. unless mandated by the CO.

I'm not saying that might not be a good idea.. but it's not within your husband's control to make that happen.  If dad is paying any bills.. he should stop like phones.

I would recommend having a talk with your DH about SS's future. Whatever happens at BM's house is her business, but the role your DH plays is his, and yours. Since SS is 17, the CO still applies, but once he's 18, things can change. I have agreed to support my kids to the best of my ability while they are in undergrad or a comparable career training program, as long as they are progressing adequately. I see it as an investment in their future as a person who will not need to be supported into adulthood. I would not support an adult to loaf, though. 

bills, provide his own place to live, etc...

The burning platform may just give him clarity and change his mind back to the service.

My SS-31... is on his way to his 14th service anniversary in the USAF. He is less than 7 years from full military retirement.

The burning platform is in large part what motivated him to launch into the service.   He graduated HS at 17.   He had his last COd SpermLand visitation.  We picked him up after his last visitation a day or two after his 18th B-day.

He came home to full adult status. To stay, he had to be either a full time student of working full time. Or half time for school and work.  Nope, he wasn't intersted.

So, we worked his ass off as our live in beck-&-call boy/chore bitch.  4mos of scrubbing, washing, folding, polishing, dusting, slicing, dicing, chopping, cutting, cooking, trimming, weeding, painting, vacuuming, etc...... 8+ hours a day M-F and he enlisted in the USAF on the delayed entry program.  We kept him on the chore model until he reported to MEPS for BMT 4mos after he signed the papers.   

He keeps a clean orderly home for himself. He is performing well in his career. He will retire from the USAF at about the time I retire at the end of my career.

IMHO, take any option off of the table other than a life of misery, or.... launching to finish growing up on his own time and his own dime.  In that situation, enlisting gets a lot of attention.  Housing, training, structure, pay.   See how that changes his choices.

You can't control what happens at BMs house (ask me how I know). Have a conversation with your husband and ask straight forward when the financial support will stop. It should be at 18, especially if your SS isn't going to school after that. Once he's 18, he doesn't have to be at your house every other weekend, your husband wont be paying CS anymore, doesn't have to pay phone, car, gas, anything. If BM wants to keep raising her big baby - let her be.

That's my agreement with my husband. Once his son graduates college (in 3 years), we are done. I bet my life he won't get a job, won't learn how to drive, won't be a normal functioning member of the society, and he will just live as a pathetic lazy leech (just like his BM). 
Not my problem as far as we don't spend a dime towards his life choices. 

She is in college but has an apartment and a roommate that she pays for with a waitressing job.  She started her first job at 16.  My 15 year old daughter has a job at Smoothie King.  They make good grades too.  They have active and busy social lives.  Remember it is our job to turn them into productive members of society.  The goal is to get them to become self reliant and that's what your husband needs to be working towards.  That is literally our jobs as parents.  Having him sit at home and play video games all day long with no goal and no direction and doing nothing is doing him no favors at all.  

I have 3 launched BDs and you have to be firm in your expectations and bring on the misery to light that fire inside them.  I also used chores to bring the misery.  Remember that bumber sticker back in The day? A$$, grass or gas, no one rides for free.  Same philosophy with an adult kid/skid. 

His future...Is not your problem.. No one until now asked your advice on raising SS when he was little and could shape him..   now when all is lost it just doesn't become your problem..  You must stop DH from throwing money at SS.  If you catch DH giving money you don't approve of. It's time for war..

I am not saying. That if SS makes the first move.  Getting a job.  Actually showing up and working as that job. For a few months.. I would gibpve him the benefit of help him out.  I don't know your financial shape. But maybe a few $ a month towards rent. Or paying some bills. For a year or so..   But again be careful. I know of some 40 yo who parents still pay there cell phone bill.