My experience is slightly different from that @CLSB - many adoptees withdraw from adoptive families completely after going through this stage. What really makes the difference between those who do and those who do not is in fact the relationship with the adoptive parents, ime, not to do with the birth family or getting over teenage angst.
He sounds very low and that needs to be taken seriously, and most definitely not minimsied as teenage angst or a "phase".
OP I think the advice from me is the same as on the other thread, which is that your ds would benefit from some really good life story work from a really good therapist who is experienced and has expertise with life story work - can you get recommendations from anywhere?
In relation to helping him with MH issues, and to help you both build a relationship, heading towards an adult relationship can you afford to go private? If you can, I would think find a clinical psychogist with suitable experience and expertise and get an assessment and recommendations.
What does he want to do about his studies?
The fact that he has talked to you is really positive. Please do keep taking this seriously and please do not think it is just teenage angst. Write down what he has said, so that you have a note of it.
In relation to him not feeling part of the family, you can turn this around to work towards a really good adult relationship with him, but you do need help to do this. You are doing the right thing getting outside help.
I hope that he gets the help he needs, my heart goes out to him 