Many older kid adoptions, if not most, are children with attachment disorders, sometimes fetal alcohol spectrum and drug exposure in utero. All of these things mess with the frontal lobe, impulsivity and can cause developmental delays. I adopted a child with substances in him and he has autism, although he is fortunately attached and emotionally stable.
I also adopted a six year old from another country. He seemed fine until his 20s when he started rejecting us. Adoption at an older age causes unusual issues that sometimes come out later.
If your son was drug affected and has delays he could need adult supports such as disability, housing, and a caseworker. He may be disabled rather than just a "bad" young adult, but that does not mean he has to live with you and cause stress and your own illness. There are supports out of your house if he is disabled.
My heart goes outvto you. Its up to you to decide what you feel would help him the most plus be best for the rest of your family. There are no definitive answers. You know better than anyone if your son can be independent but even if he cant be 100 per cent there is help for him and he can reach his highest potential and leave the nest. Some adults need a group home...my son with autism has an apartment and part time job and ssi. He rarely needs us, but I am his payee. Yes, he has his own money and is happy and loving and content. My other adoptee whe came at six is very finsbcially successful but very angry and we dont know why. We never see him...his choice.
Hugs for your hurting heart.
