I have two children that are on medication for ADHD and loss of appetite is definitely a worrisome side effect. The older one seems to have outgrown it now that he's a teenager, but it is still difficult to get the girl to eat and she is very slim (her sister that is 2 years younger outweighs her and she is not overweight by any stretch of the imagination). We too were concerned and the pediatrician blew us off so we took her to a pediatric psychiatrist for medication management and with some adjustments have seen improvement.
I have a friend who's son takes medication in two doses. He has an 8-hour dose that he takes to get through the school day, then he comes home and eats dinner as that one wears off, then he has a very low dose short acting one that he takes to do homework, etc., for the rest of the evening.
There are many different types and combinations of treatments. If your doctor is not addressing your concerns, find a new one for a second opinion! If you are seeing a regular pediatrician, switch to a pediatric psychiatrist. They have much more experience and greater expertise with these types of drugs.