At that age, there is no limit!
What you can do is keep some snacks around. He needs to eat a good breakfast every morning (a bowl of cereal won't suffice - cereal and a couple of slices of toast, or an egg, or something, that's fine). He also should then have a snack (like a sandwich) between breakfast and lunch, and another between lunch and dinner. Then, for dinner, he needs a good one - not just salads. Yes, he needs meat - we're meat eaters, you know! It can be healthy - chicken, fish, pork, lean beef, etc. But, meat is protein, and he is working out and growing like crazy, so he needs that to keep functioning.
Then, he'll need a snack between dinner and bed time.
Just keep some healthy snacks around (fruit, veggies (I like broccolli and ranch, or celery and blue cheese dressing) - man was not meant to subsist on three squares a day!
Now, I'm hungry, so I'm gonna go get a snack!