I don't want my son to feel that pain, and I don't know if there is anything I can do to stop it if he does.

I don't think there is anyway to stop these things from happening; sadly, it's a part of growing up. All you can do is be there for him when he's down, you can't stop the pain - but you can soothe it. Fortunately, Charlie seems to me to be a pretty upbeat kid, and that should make things a bit easier for him. Kind of like the difference between my oldest and youngest. The oldest takes everything to heart, takes hurt feelings very hard. But my youngest is more like Charlie, very upbeat, it takes alot to really faze her.