If the doctor blows it off... find another doctor. Plain maybe, simple... not always... Necessary, yes.
While I have no experience with children like yours, and I am sorry you are having such a terrible time, I will give you an example of why finding another doctor, one who will listen to you as a mother, is so important.
A friend of mine has a daughter with ADHD, she was put on a new medication, actually the medication is an old one that was used for high blood pressure, but is now being used for ADHD. Well, Sara (9), the daughter, started taking the meds... she became tired weak and lethargic. She began sleeping 12 hours a day and that gradually increased to 23 hours a day. Yes, she was only awake one hour a day. Her beautiful auburn hair began to turn gray.
The doctors tested her for anemia, then mono... eventually they began testing for leukemia. The mother stated all along that she thought it was the medication, but her ideas were refuted by the doctors as this medication carries very little if any side effects.
This went on for almost 8 months. Mother eventually did her own research via myself and the internet. She found another doctor at a local children's hospital, took the info that she had collected on the medication and Sara was immediately taken off the med as her blood levels showed that she was near death. Within 24 hours Sara was a healthy vibrant child of 9, acting like every other 9 year old girl and fighting to go to bed, when it was just days before she fought to stay awake.
As a mother you do have the best instincts and you should find a doctor who will listen to your instincts.
I don't know if you have contacted your local chapter of NAMI, but they may be able to help you find someone who is right for you and your son.
NAMI: National Alliance on Mental Illness-The Nation's Voice on Mental Illness-Formerly National Alliance for the Mentally Ill