what would you do if you have a very serious brother?

I would first try getting him out more often and making him socialize. If he resists that or doesn't socialize well then try counseling. He may have a social disorder, that keeps him too serious. He may need to be on medication for an imbalence of some kind. If he doesn't lighten up with increased exposure to fun, then check for medical problems and barring that I guess he is just going to have to deal with it.