Or treat it like a new gun purchase........give him the price range and take him shopping. Let him pick out what he wants......with dad's pre-purchase approval of course. If he doesn't like the knife you pick out for him, he might not carry it which is just as good as owning a gun that you leave in the safe all the time.
Great advice here.
Personally, I think he'd be well served by a Swiss Army Knife (SAK), but if he doesn't care for it, he's a lot less likely to carry it.