Ask him, or better yet, take him shopping where there's a big selection.
My boys will be 16 and 13 shortly. They've always been into knives, probably because their father, uncles, and grandfathers are into them... and because we use them as avid outdoor enthusiasts are wont to do.
Once I took the boys to Cabelas. They got to pick out a knife. Oldest picked a Kershaw Brawler, youngest picked a Kershaw Scallion.
Once we were on vacation in Oregon, and went to a sporting goods store. They sold Balisongs in Oregon, which intrigued the youngest. He got one. Oldest wanted a Gerber thing that looks like a money clip.
Christmas, youngest wanted a Ka-Bar mini Marine Combat knife. Probably saw one in a video war game he plays. Whatever, OneStopKnifeShop filled the bill. Oldest wanted a Leatherman Wave.
My point is, I could have come on here and asked the same question, and got a hundred GREAT recommendations. But in the end, my teenagers knew what they wanted, or spotted it pretty quick and decided that it was what they wanted... and it was NOWHERE NEAR what I would have guessed, or had recommended by you fine folk.
If he chooses some sort of Chi-Com Mall Ninja crap-tana, then be a guiding light of reason and wisdom in assisting with his choice. But let it be his choice.