I would start with your family doctor; or if in school, his guidance counsellor. Either or will have the ability to make a judgement call on whether your son needs some professional help and what that sort of help ought to be. They will then make a referral according to what they deem would be appropriate treatment. It could be to a psychiatrist, psychologist, or some other such therapist.

Personally, if it were me, I would be inclined to allow them to deem what is the best measure and course of action than taking it upon myself to make the decision. The doctor will have resources as well as their own professional relationships with those in the mental healthcare field and as such have a bit of an idea for matching up your son's particular issue to the ideal mental healthcare provider. I repeat then that it might be best for them to consult with professionals to determine the best treatment or course of action. Of course too, any good doctor would also consult with your son.

Incidentally, don't expect to be kept in the loop about this as your son is an adult and legally in charge of his own medical choices. Even if you asked, the doctor is not bound to do so. Basically it would come down to the young man's directives.