It's not out of the range of normal behavior, no. Kids that age are exploring their bodies, and he's not old enough to have any sexual association with touching his penis. You probably want to talk to him about it, though, and let him know that's something people do in private. No scolding, because you don't want him to feel bad about touching himself, but he may need reminders when you see him doing it. Kids that age aren't really aware of what is socially appropriate.