I’m a little biased considering I recently started shooting USPSA, but if you’re looking for a pistol related shooting sport I would recommend USPSA. At 16 he shouldn’t have any issues running the guns (I was pretty deep into competitive trap at 16) and the movement, speed shooting, and different stage presentations are going to make for a more interesting match in my opinion. There are also age based classifications, so he will be competing against shooters in his age range.
Just a warning: as a father/financier in chief, if he really gets into it, prepare your wallet. Shooting sports can get VERY expensive. Guns and gear are one thing, but the ammo to sustain a competitive shooting habit is where the real costs start appearing.
For USPSA, most any pistol you have that is 9mm or larger will work. “Limited” division is probably where you’ll want to be starting out, but just be aware that the division also allows for some highly modified guns as well. From what I’ve seen it’s hard to be competitive in Limited with something like a stock Glock, but it’s still a great place to learn (and have fun!)
You can use something like a stock Glock to shoot in “Production” division, but that limits you to 10 rounds per mag which means he will need to get good at mag changes AND (more importantly) stage planning. Starting off I think it’s easier to just start in Limited so that you can have large mags and get used to just running the stages. Then worry about potentially moving to limited capacity divisions.
Another note - if you want to buy a gun for USPSA, I would recommend getting one that has the ability to accept an optic as that gives you the most flexibility. Something like the Glock MOS, Walther PDP, etc. You can start off just using irons and compete in Limited (or Limited 10 or Production), but all you have to do is add an optic and you can compete in the “Carry Optics” division too (which at least in my area is really big).
Also take a look at the USPSA rule book for the weapon division rules before you buy guns/gear (things like allowed modifications, mag length, etc). It’s a large rule book, but the appendix with the gun divisions is pretty straight forward. Had a guy show up to a match who was really excited about his new 33 round Glock mags until he learned that they weren’t legal for the division he was shooting in.