If you think your son is always hungry now....
Just wait until he's 14.
My son has always gone through phases where it seems there's never enough food in the house and then he will barely eat anything for a few days.
I always know exactly what's coming....All the sudden he's another inch taller and none of his shoes or clothes fit.
For this reason, you need to establish healthy habits now. Aside from breakfast, lunch and dinner, my kids were always allowed to have any fresh fruit or vegetable snacks they wanted. If they ever said they weren't in the mood for celery or carrots or melon or a banana, then I figured they weren't really that hungry and they could wait for dinner.
I have friends whose kids are so fricking picky. They've never tasted a brussel sprout or a bell pepper or string bean in their life. I tell them if their kids get hungry enough, they'll eat a dirt sandwich and be happy to have it.
What it boils down to is, is your son really hungry?
Make sure he's drinking plenty of water, make sure he's getting plenty of exercise.
You say he's not overweight, but he's obviously not underweight either. So he's not starving. You just don't want to get in the habit of him needing to eat something everytime he says he's "hungry", because it could be that he's feeling something else. In between meals you can offer him orange slices or celery with peanut butter. If he says that's not what he wants, then I would guess he's not really hungry.
You could try giving him a little more to eat at a time.
On the mornings you give him eggs and toast, also give him some yogurt or fruit. Mix it up a little. I have to be honest with you, my daughter loved oatmeal but she got so burnt out on it. Try cream of wheat with brown sugar and fresh berries. Try Malt-O-Meal...that was my father in law's favorite and my kids loved it because Grandpa did.
If your son loves, eggs, try scrambling them with a little cheese and mushrooms, maybe some tomato and avacado. Always add a fruit, cottage cheese or yogurt.
One thing my kids loved was "egg in a hole". Passed down from Grandma.
Take two slices of bread and use a cookie cutter or round glass to make the "hole" in the bread. Butter both sides of the bread, including the separate hole shapes.
Use a little non stick cooking spray in a frying pan, heat, and lay down the pieces of bread. Crack an egg into each hole and allow to cook, then flip to the other side. Put the little "hole" pieces into the pan to "toast" on each side. Serve with fruit and/or yogurt. He might just be getting tired of having the same old things so he's not feeling satisfied. Pay attention to things if he says he craves them, it could be a sign of a deficiency of a certain thing like protein or a vitamin or mineral.
If his blood sugar drops he could be feeling the need to eat something, so offering the healthy stuff is your best bet.
I hope you get some great advice!