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.