Hi, my son is 10 and he was on every single kind of medication out there for ADHD and responded to none so I took him off and he is getting an assessment done (As I have Bipolar disorder). He was worse on the pills than off....mood wise.
But he lost a ton of weight....he was always being called chubby before he started concerta and adderall....now everyone is worried he is too thin.....so yes it does suppress the appetite. That is why there was a whole controversy of women taking their children's ADHD medication so they could do their house work very fast and "diet" at the same time.
Now that my son is off, he eats like a horse again but all very healthy food...so I am not worried.
I totally understand your concern with your son's weight loss on ADHD medicine. My 9 year old son has been on Concerta for three years and the first year and a half he was very thin. Now he is the right weight for his age and height. He is on 18mg. We tried him on the 27mg but it was too much. He was thin for that period of time, but he has jumped two grade levels ahead of his class in reading and has much better impulse control. He eats a good breakfast and eats like crazy between 5pm and 9pm. He doesn't eat much lunch because of the meds so I make up for it on the other meals.