buy the right size bike for his proportions. if a 24" bike fits him now, it will be fine for now. if you buy a nice bike and take care of it, it can be re-sold and the money put toward his next bike. if you buy a 26" bike, you will need to get one with a frame small enough for him and he'll out-grow that too. bikes with 26" wheels come in frames designed for riders from under 5 feet tall to riders close to 7 feet tall, the difference is the frame size, not the wheel size.