I flew the GB Cub with an OS 40-4c. As long as it is built light, the 46 will do fine. My current H9 Cub 80" has a Magnum 52- 4c. Some pilots will say don't use anything less than YS 91. With their scale speed of 250 knots, it take away the relaxing flight of the cub. I prefer to let the plane fly instead of jerking off the ground then throttle back to 1/4 using a larger motor.
I'd suggest the Evolution 46 for your step sons plane. An easier trainer motor than the 4 stroke. The engine is already preset and ready to fly. The OS line of LA's are a good choice also.