It's been said a few times already, but the BK14 with new grips would be perfect. Even a 14 and let him choose some paracord to wrap it in, so it's distinctively his own!
My son is 6 and he has a slipjoint he can carry around when he's with me. He's nuts about knives right now and the 14 is actually really good for him. I keep mine pretty sharp, since I carry it with me everywhere, and with some one-on-one time he quickly learned to be conscious of knife safety. Kids will suck that stuff up, especially if they get the privilege of owning their own knife. Our agreement is that as long as he follows the safety rules we've put in place, he can use knives. If he gets careless and goes against the rules, the knives get taken away.