Maybe take him to the vet to get a weight check and an exam. If vet ok's him, try mixing his food with gravy, cottage cheese, or plain yogurt. I think 2.5-3 cups is average for his age but if you are uncomfortable with how skinny he is, definitely get him checked out. Ribs should not be prominent but you should be able to feel them. Can you post a picture from the top view of him (back) so we can see how skinny he is?