To be honest, I'm taking the opposite tack. My first knife at 6 y/o was an SAK classic. No lock at all. I eventually got a lock-back, and I have to say, most of them were stiff initially. Which, lets face it, is what the lock is supposed to do- keep the blade from closing.

The big thing with a lockback is, yes, your fingers get in the way as you unlock it. which is why you don't have pressure on the blade when you do it. I've only been bitten by a knife once - cleaning caulk out of a door, when I was 26. And that was when it slipped, not closed, into my thumb.

I really think you're probably not giving your son enough credit. then again, I don't know your son... Even if he does catch a finger, he'll learn a valuable lesson.