Any kid vitamins

Ladies my little brother is 10 almost 11 on March 31. He hardly eats anything he is so picky he always wants pizza the one you put in the oven. Lately he has been only eating the cheese and pepperoni. We're worried that he isn't eating that much and what could happen if he doesn't eat enough. Are there any vitamins that will bring up his appetite and make him want to eat?? He's short for his age and he isn't starved looking. Should we get him checked out by a Dr?