My daughter was just tested the other day for ADHD at our HMO's psychiatry department. It was a 3-hour long battery of tests where they assessed multiple children at once. All of the parents had a class during that time where we learned about the latest research on ADHD.
So, what I found really interesting was that the cutting-edge research on this shows that this is actually a structural brain issue. A breakthrough that researchers had about 3 weeks ago was an extensive study of behavioral therapy (rewards, etc) vs. medication only vs. medication + behavioral therapy. Apparently, kids who received medication only did just as well as those who did medication + behavioral therapy, and both of those groups outperformed behavioral therapy only. So... bottom line, medication is really the only thing that will make a dent in ADHD, according to the research, and behavioral therapy simply isn't as effective as previously thought. This is most likely because ADHD itself is caused by neural transmitters not transmitting information to certain areas of the brain. These areas remain "asleep" when they should be receiving and processing communication. Stimulants help "wake up" these parts of the brain. No amount of behavioral therapy will accomplish this (again, verified by peer-reviewed research). It is a brain structure issue.
Something else the psychiatrist told us was that ADHD is almost totally unpredictable. These kids will go for a period of time and be a lot better, and then suddenly be a lot worse, seemingly without any rhyme or reason. He did say that Omega 3 fish oils seem to be promising as a dietary addition, but other than that, nothing has really been proven to help in terms of diet.
As far as the medication goes, it is metabolized very quickly through the system, so for most kids, they take it in the morning (say, 8am), and by 1-2pm, it is completely out of their systems. There's not a whole lot of downside to taking the medication if your son hasn't had any adverse reactions to it so far.