Hi, I'd like to help you with your question.
It sounds like your son either has Cyclothymic disorder or Bipolar disorder. It is hard to know unless he receives a full evaluation by a qualified therapist or a psychiatrist.
It is going to be difficult to convince him that he needs to see a doctor or therapist unless he feels the need to do so. It is very common with people who are diagnosed with mental illnesses such as Cyclothymic or Bipolar to shun help, even those already receiving treatment. They will go off of their medications because they feel better or are on an upswing. Then it takes a hospitalization to stabilize them again. Even therapists struggle with helping their clients who prefer to ignore treatment.
You are not alone in your frustration with trying to get help for your son. The issue is that the law prevents anyone from forcing another person into treatment unless that person has threatened to hurt themselves or someone else or are somehow in so much distress that death is a possibility.
What you can do is help yourself. The calmer you are and the less stressed you are, the better off you will be to be there for your son. You can try some of these resources to get started:
http://www.nami.org/- NAMI is a nation wide organization that advocates for the mentally ill and the families involved.
http://www.nmha.org/- Is another group that can help you find support and assistance with your son's situation.
Also, consider therapy for yourself. The support and insight can help you feel better about your son's illness. The therapist may also help you find ways to work with your son to get him to join you in therapy. Maybe once he sees you going, he won't feel so hesitant to try himself. Talk with your doctor about a referral or if you attend church, your pastor can help. Also, try searching on line at http://therapists.psychologytoday.com/rms/.
Here are some books that may help you as well. One is called When Someone You Love Has a Mental Illness - Paperback (Nov. 1, 2003) by Rebecca Woolis. Another one is The Burden of Sympathy: How Families Cope With Mental Illness by XXXXX XXXXX Karp. These are available on Amazon.com or your local library may have copies for you.
I hope this has helped you,