kaos - IMO, get the big barrel. It won't turn a bad hitter into a good hitter... but it will reward a good hitter and give him more pop on his drives. Big Barrel bats typically have larger sweet spots than other bats. The newer composite big barrels have very big sweet spots.
Kids need to experience success to continue to work hard and enjoy baseball. This is especially true if you are playing competitive baseball. You can use whatever you want in practice such as wood if you think it will make you a better hitter. But if your son comes up to the plate with the game on the line, all eyes are on him, do you want him to be the hero or the goat? Are you thinking about HS baseball at that moment?
When my son was playing U13 travel ball a few year back some parents had their kids swing a -3 "to get Johnny ready for HS". Those kids mainly struck out or grounded out. Today, the ones I kept track of are sitting on the bench in HS or not even playing any more. Those swinging -5 or -8 bats were way more successful and today they are also successful in HS. Going to a -3 was a breeze. Teach success and that is what you will experience later on.