I would start keeping a diary of what he is eating and when he complains. We recently took my daughter off of wheat/gluten. She told me a few weeks later that her tummy used to hurt all the time, but now it doesn't hurt at all anymore. I was shocked and amazed. I had no idea. A hurting tummy was her normal, poor thing. Your son may be having these types of reactions. But, it could be that he is full, or hungry, or any other number of things. When I started noting what my daughter was eating and her reactions, I saw a strong correlation with breads/pastas and her issues. We eliminated them, and the next day she had a strong reaction (vomiting and a rash), the day after that, she was perfectly fine and healthy, and has been ever since. She's 7, and it pains me to think she has suffered for so long without my understanding her issues.