How is he now?
If he is having trouble breathing, such as he can't speak a full sentence, then take him to his doctor or the emergency room.
There is a lining in the lungs that allows the tissues to "slide" freely. Sometimes after a trauma that lining is damaged and it can cause "pleurisy." The lining sticks and it gets hard to breath. It may not show up for a few days too.
Basically though, kids are tough. Just keep an eye on him for a few days.
Hope it helps.