Hi, I'd like to help you with your problem.
You said in your description that your son has been using marijuana and some other drugs. He also seems hyper in his behavior, has trouble concentrating and is agitated and is having trouble getting along with others.
You mentioned that you are worried that your son has manic depression or bipolar disorder. There are numerous symptoms for both disorders. Although your son has a few, it does not appear he has the symptoms to qualify for either disorder. It is difficult to tell for sure since I cannot do a complete evaluation on him, but some of the major symptoms you did not mention such as persistent sadness, loss of interest in hobbies, or thoughts of suicide. Also manic depression symptoms include grandiose thoughts, severe insomnia, and inappropriate elation. You would see a more dramatic change in his behavior if he had either of these diagnosis.
You can suggest he see a psychiatrist. He could also see a therapist for an evaluation then determine if seeing a psychiatrist is indicated. Since he is also using drugs, they may affect his behavior in ways yet undetermined. That may explain some of what you are seeing with him. The fact that he is 20 years old and is now an adult may hinder you convincing him to see a doctor. He ultimately has the say so in what he decides to do. The only way you could force him to get help is if he shows intent to hurt himself or someone else. In that case, you can have him involuntarily committed.
Talk with your son, express your concerns, and suggest he see someone of his own choice. You can make suggestions, but at his age, he may see it as an attempt to control him and may refuse to comply. I would speak to him in a nonjudgmental way and let him know that you care for him and are concerned.
You may also want to contact a local Al non program for support. If they cannot help, they will refer you. Your local community mental health center will also be able to give you referrals appropriate to helping your family through this. They will also help you find ways to help your son.
There is also a book that can help called Tough Love (Revised Edition): How Parents Can Deal with Drug Abuse by Pauline Neff. It may address younger children but some of the information could be altered to use with your son.
Remember that you need to care for yourself as well. It is hard to parent a child with issues, but the support you offer him and being there for him will help him find his way.
I hope this helped you,