my first, at 6, was the typical SAK classic, you know, the key chain knife.

Buck makes a half model 110 I think. Lockback, strong, kind of heavy.

Spyderco or their cheaper Byrd knives have some small models that may fit his hand.

Case knives are classic.

Personally, I'm not a fan of locking knives for new users. Typically you have to hit them "wrong" to close them, so keeping the threat of cut fingers will help them forge better knife habits. I've only ever cut myself on a locking blade.