You must mean 24" Wheels, at 24" frame is pretty huge.

However, you may just have to take him around to test ride bikes and see what works. Also, as long as the pedal brake is working he should be safe until he grows enough to grasp the hand-brake. Most hand break levers work the same and are about the same size so it might just be something you'll have to work around until he gets bigger.

The other option would be that when you go to your local bike shop maybe you could talk to one of the mechanics and see if there is any way they can modify the bike or if there is an aftermarket part that can make it work.