The school system can't force you to medicate your son and I'm surprised that they are so adament about it. Is his behavior that 'bad' that they can't handle him otherwise? Or is his teacher too lazy to help him learn in an effective manner.
My son is 12 and we just recently put him on ADD medication (a very low dosage of Vyvanse). We were recommended to have him evaluated twice when he was younger and did go to his pedicatrician but it wasn't until he was about to push me over the edge that we decided to try meds. I think other things should be tried first including good parenting and teaching methods (the teaching part might be the issue) and diet adjustments. Hopefully if you're that concerned about what is going into his body, you'll take the time to watch what your son is eating and how it affects him. Our son no longer is allowed chocolate milk, sodas other than Sprite on the weekends, no gummy treats (which is really candy with LOTS of dye), etc. Different things affect different kids but I know this is a big one. Our son always had the most issues at school right after lunch (he always drank chocolate milk!)
It just got to the point for our son that even though he wanted to do good in school, he just couldn't stay focussed long enough to learn and pass his tests. He was getting very discouraged with his school because his grades were so bad. He had managed to keep up before but his school is very academically demanding and he just couldn't 'get by' anymore.
The other thing I would suggest is taking him to a psychriatrist or psychologist of YOUR choosing and someone who specializes in children. Someone who will ask him questions and try to figure out what exactly is going on and not just assume it's ADHD. I think that teachers and school counselers sometimes try to play doctors and diagnose kids even though they don't have the medical education to do so. A psychologist might fit you better since they don't prescribe meds and might not be so willing to just send you off to your ped. who will. There are behavior assessment tests that a psychologist can do but most of the time your insurance will not cover it so just be warned if you go that route, you may have to come out of pocket.
And as a side note, Vyvanse which is the drug my son is taking, is a capsule that can be opened up and dissolved in a drink or on the tongue. And even though our son is on the lowest dosage, we gave him 1/2 the pill for the first week to help him adjust and to make sure it didn't cause any weird physical side affects like heart palpitations. The two side effects our son has is the lack of appetite (like you said, diet pill). We make him eat breakfast and then take the pill and even though he'll eat something for lunch and dinner, about 8:30pm he's all of a sudden starving. We let him eat then but that's also because he's skin and bones and can't afford to lose weight :)
The first few weeks were also a bit rough on his sleeping. He just has a harder time falling asleep. Once he's asleep, he's out for the night. The dr. gave us an allergy pill to give to him but that didn't help so I've switched to Benedryl. He was already taking a Claritin every day so I just stopped that and give him a Benedryl a couple of hours before bedtime since it makes him tired. I figure, at least Benedryl has been around for ages.
Anyway, good luck with all this and keep up posted on the outcome.