Need help for my son!!

My son is on ADHD medication. He does very well on it. He was failing every class at the beginning of the school year and now he isn't failing ANY. I have to admit I was skeptical at first when the school called me in and told me that he should be seen for ADHD, but I'm so very glad I did. One problem.. he is loosing weight! Not much, but enough to concern his doctor. 1/2lb in a month. His doctor called me today after she reviewed his weight check last week. But he EATS A LOT. Like 2 times the amount I do. He likes everything and he's not picky. Always askes for more, but he is loosing weight?.. So what should i do? Pedia sure/ensure/nutritional drinks aren't an option. My doctors advice is to make sure he's eating most of his meals.... He eats literally more than me, so that advice was no help lol.