Go to a few bike shops and learn a bit. Then, look for a good used mtn bike. Us mtn bikers get upgradeitus and usually sell our old bikes for much less than they are worth. For your application I’d think you wouldn’t need a full suspension bike so maybe look for a hardtail with a decent fork up front. I’m now riding a full carbon Santa Cruz 5010.
+1
No substitute for you and him going on a full day road trip to a town where there are bike shops, ya gotta see for yourself. Advice from bicycle people with actual bikes on the scene to try is worth a 1,000 even good posts on here.
If there's a college town go there, there will be a bunch of bike stores around a college and this time of year they should have a bunch of used bikes. I agree with others, you want "hardtail" mountain bikes with front suspension.
You need to go to a bike shop too to find a bike to fit your son.
However you get bikes, do not purchase a bike without also buying a tire pump, spare tubes, and tire levers to remove the tire from the rim. This will save you a long walk and a second trip to the bike store
