For what it's worth, I would suggest Power grips. Aside from the question finding shoes/ growing out of them and (above all) adding to the risk of a fall while he gets used to clipping in and out, there's the fact that kids (at least my kid) doesn't really like suiting up for a ride; he just jumps on and goes.

I used power grips myself until about two months ago and don't frankly find the SPDs that much more efficient; or at least I don't really notice much more efficiency. Except on the trainer, where they're great.