sounds like depression, is there any other symptoms, i e talking to himself?
may be an underlying mental health issue, my brother was perfectly fine up until 19, he is now 30 and is diagnosed a schizophrenic but my mum refused to see a problem despite all the warning signs, and didnt take him to the docs until he was 26......if she would have acted sooner, he may not have been as bad.
good luck, hope its just 'simple' depression but id go to his doctor alone first and express your concerns, the doc will then ask for you to arrange an appointment with him there, this may takes motnhs to get him there especially if he does not see a problem... imagine now in your 'sane' mind someone trying to convince u there was something 'wrong' with u....baby steps and patience is the key
hope it all goes well for u xxxx