9 Year Old Hitting Help

I have a U9 boy in his first year facing other kids pitching. We use the "soft" balls for games. During hitting practice I like to use tennis balls, game balls, and regulation hard balls (especially for the kids who also play travel or will be moving up next year). My 9 yr old swings aggressively at the tennis balls and "...wants to nail it". His swing is noticeably more tentative with the game balls and he refuses to do nothing but bunt when hitting the hard balls. He claims the harder balls hurt his hands (he uses a cu31 bat). Any suggestions? Is this a grip pressure problem? He is not afraid of the ball since he has been hit by pitches a few times with no after affects and does not bail out on inside pitches. WOuld a wood bat help.? If so, does anybody make light wooden bats. His bat is 29 in, 19 oz.