8yo oldest son is very aggressive at home w/siblings. He always takes biggest/best piece of anything, always is first and pushes brothers aside. But at school he is shy and quiet and always takes the "back seat" to the other boys. If he is next in line to kick the ball and they push him aside, he moves over, or they take the ball. Why is he not agressive there too? He also complains he has no friends. How do I help him?