As an umpire, I can tell you that there is no real regulation in place, but it does depend on your son's age, and the league he plays in. If he's still in youth programs, either one is acceptable, the only difference is that the hockey style mask is actually more dangerous, since it wraps around the head. If he were to get hit by a foul ball, the hockey style mask could easily shatter if the ball hits it in just the right spot. Furthermore, the hockey style doesn't do much to prevent concussions, even though it might appear to do so at first glance. So, it doesn't really matter, but if you're concerned with safety, I'd go with the traditional. But, bring both just in case, because there could be some league specific rule that I don't know about.