Are you really sure your son has ADHD? Since the Ritalin and Adderall seem to be causing problems and not improving your son's health, I wonder if the doctor is treating the wrong disorder. By the way, what is Clonitin?
PDD-NOS can have some ADHD - like symptoms. You are saying that teachers are complaining - does he have any help at school? Are they teaching him to sit down and pay attention? In my child's school (he is 4), they use behavioral methods in order to get him to "sit and concentrate" - reward systems, choices, fidget toys. A very good psych I know says that effects, long term and short term of all these meds, are not studies in young children.
What is his main ADHD issue? Aggression? Concentration? What about PDD? I am a firm beleiver in behavioral methods as they seem to work, it just takes long time