If all he ever does is ride around the neighborhood, the X24 will get stolen before he could ever outgrow it. Whack a longer post in the frame, and he could ride it to college. The only issue he might have is reach. Make sure that with the bars in line with the fork (side view) he can still reach the grips and operate the brake easily. From my time selling bikes, I can assure you that height is secondary to reach. Many times, kids simply cannot effectively use their brakes, even on a bike with ample standover room.

The majority of responses you got are from kids that ride 20" bikes. I have both, and can assure you that even if he decides to get into tricks and jumping, you'll still get plenty of use from the X24. Worse case is that in a few years, you have to buy him a 20", and then you can ride the 24" with him.