The kids and adolescent forums would be fine for him.
Perhaps you could supervise him when he signs up, to make sure he enters his correct age. That way he will not have access to the Adult Forum, the only bit of WP that would be inappropriate for him. I didn't put in an age when I joined, and it wouldn't let me into the adult forum, so I simply went back and added my age which is fine for me because I am 26, but a younger person could have added an age to make them appear over 17 when that was not the case. I'm sure your son is honest and sensible at heart, but it's normal for kids to want to go where they aren't allowed!
WP is a nice, safe place. Nobody will bully your son here.