JMT, I've hinted at it before, but I'll just say it now:

Your DH has always been a weak parent. The only reason he seems strong is because you've ruled over the house with an iron first and made sure everyone behaved appropriately. You don't take any crap from any of the kids, and they don't tend to deliver much in return. It's really easy for your DH to be a "strong" parent when you're the one who instilled the values and enforces them on the daily. YOU keep the household running and everything afloat while he does his job and backs you up.

Remember, YOU were the one contacting doctor's offices trying to get records of BM's lies and SS's diagnoses. YOU were the one in contact with the prosecutor about BM's forgery of HD documents. YOU were the one in contact with the attorney about contempt charges, etc.

YOU have been the head of everything for a long time. It was REALLY easy for your DH to agree to all of it because you were acting as his "personal assistant". He has never had to be the one thinking of consequences or therapy options because you had it all lined up and ready to go for him to choose the best option. And since you would enforce any of them, he never had to deal with any of the blowback.

This is the first time that he has been faced with you dropping the rope and leaving it up to him, and he failed. He couldn't do it because you've overfunctioned for him for so long. He doesn't have the muscles to figure it out on his own, only to backup what you do. And he's not used to you being angry at him because you always have a plan. This trip was a wake up call for him that he actually has to put in the effort, and he has no idea how to.

I'd bet money that if you stopped trying with SS, so would your DH. He kept the relationship alive because you laid out a golden path for it. Oh, sure, he'll blame SS being older, BM blocking his attempts to call him, etc. While that's partly true, your DH will also stop because it's hard and he doesn't want hard. He'll justify it in his brain that he already has it rough with OSS's diagnoses, having to work 24-hour shifts, two stepkids at home, SS living so far away, etc. But it just comes down to it being hard work on his own - hard work that you've historically picked up for him.

Keep the rope on the ground. You're going to keep overfunctioning for OSS because he lives there, but you can 100% drop the rope for SS16. Your DH will either learn how to pick up and carry the rope himself, or he'll let it lay on the ground and move on.