Was it mainly the steeper black run that your son was having problems with the bigger board, or all the runs in general?
My 12 y.o. son is 4'10", 95 lbs. First two seasons rode a Burton Custom Smalls 131. He probably could have ridden it again this season but his feet are too big (size 8 mens). Got him a Capita Stairmaster, and I considered buying a 144 but ended up getting the 148--figured that it would last at least two more seasons and in that time he will probably have a growth spurt and end up bigger than I am (I ride a 147). First time out this season, I thought he'd be in for a day of struggling with a bigger board but he took to it no problem, was riding better on it than his old Custom Smalls. He says it's actually much softer than his kids board, much easier to control and more fun to ride.
Unless a kid's competing on a team and needs specific equipment, once they are at a certain level of proficiency on a board (linking turns, going down blues), they're pretty adaptable to whatever gear they have.
No actually he tried it on the bunny hill first and couldn't make a turn without catching an edge and falling...this was with an instructor too. He took his Chopper down the black diamond run but again that is a steep run that is often moguled out so my guess is he just side slipped it.
You've given me some hope...I was after the same thing going with the 144 from the 130 (1.5-2 seasons of use). The post office has my new bevel guide so I will bevel the edges of the Capita tonight and see if that helps. Honestly I have already ordered the K2 Vandal 137 but it's easy enough to return....plus it's only money, I'm supporting the economy right? LOL