T.S.
If he's at a healthy weight why are you concerned? If it's healthy cereal I would let him eat as much as he wants. And if he makes himself sick, well then he will have learned a valuable lesson,right?
If it's a cost issue then buy him his own box and tell him it needs to last for the week. I do this with my youngest. I buy a fixed number of lunch "treats" and she knows if she decides to eat them after school of in the evening then she won't have any more for her lunch until the following week. It makes her think first and learn how to stretch it out (she is impulsive and ADHD too.)