Check out the book "How to get your child to eat, (but not too much)". Lots of good info there.
Stay out of power struggles over food. Watch him. Is he alert, cheerful, and active? Is he peeing and pooping? Does he look and behave like a healthy child?
If the answer to all of these is "yes", then don't worry. There are times I could swear my child photosynthesized, because she could not possibly be running her body on what she was eating. But she did! Kids go through various stages in food.
If your child appears listless and unhappy, or is not peeing and pooping, get him to a doctor ASAP.