If you’re looking for something smaller I’d try to find a Diana 24 or 240 ($215 plus shipping in Nutz Airgun FS), a Weihrauch HW30 or HW50 (usually $275++) or an old BSA Meteor ($40-150 at gun shows) but watch out because the newer Meteor Mk7 models are Gamos which I’m not a fan of. All of those will last him his whole life and his kids lives as well with minimal maintenance.
For new, easy to get with a scope I’d look at the Ruger Redhawk. Cabelas has them listed for $200. The Ruger Redhawk is a copy of a Diana 34 so parts/accessories are easy to get (like a full power spring if he gets his PAL)) as the design has been around forever. The only drawback is that it’s a full sized gun.
Personal opinion time. I like guns with real springs in them. I’ve owned gas piston guns and there’s nothing wrong with them but I’ve never fully trusted the technology even though it’s been around for 30+ years. People say they shoot smoother but I‘ve had, and still have, spring guns that shoot just as smooth with some tinkering. Also if you’re looking towards long term I’d go with a wood stock but that’s me. Finally, given the choice I’d want a 22 shooting 495 FPS as you can use it to hunt small game. It’s hard to find a cheap quality gun shooting sub 500 in 22 I’ll admit but if you pick up any of the guns I’ve listed you can always get a 22 barrel later if you want.