Has he ever had assessment for neurodivergence?
What's the relationship like between his mother and his father?
Was the divorce difficult and how old was he?
You have children, what, 4-5 years younger than him at most and possibly closer in age? Assuming they are you and DHs? So he was quite young when his dad and mum split? (maybe they aren't DHs kids of course)
His dad works late and doesn't seem that bothered about either his behaviour or how he is feeling.
Kindly OP, you've been lucky not to have your own children who have had big difficulties before now. Lots of us have had to radically rethink what parenting is, and pick our battles. I was once like you, wanted everyone to come and eat nicely downstairs, wash, brush their hair, go to bed at ten, get up for school. Two autistic children with regular meltdowns and burnouts meant I had to let go of my ordered life and parent the children I had.
I feel sorry for you tho as he's not your son, it seems to have all dumped on you at once, and your DH doesnt seem very engaged.
I think you and DH need to look at SS as someone who needs your help, work out what you can do for him, yes set boundaries, but also give quite a lot of energy to working out what's going wrong for him. Support and help him.
You might, unfortunately, need to live in a way you don't want to live for a while. Your own children do need to be supported and protected from anything dangerous, but you might have to have a dirty room, pizza smells, chaotic bedtimes for a bit.
If it was as easy as saying to someone in this state "You have to wash" , the problem would be easy, right? You know already this young man has more deep seated problems. Help your DH help him.
Sadly if your DH doesnt want to do the hard yards, your marriage might be over anyway. Do you think he'd help you if one of your own children went off the rails?