There are many best rifles, it all comes down to personal preference. If I could give you my best recommendation I would look into a Gamo air rifle. It's a Spanish brand, very big in Europe for their high quality productions. Also a very innovative company I must say.
Gamo has a variety of hunting rifles which you can find on their website.
Pretty neat, they offer different calibers: .177, .22 and even .25! Not sure which model, but there's one that puts out a .177 at 1600 ft/s ... dropping boars. Though, achieved with PBA pellets which are their non-load alloy pellets making for a higher velocity. Pretty amazing but might not be necessary for your son's use. There's lots on that website, just have a browse.
Maybe a fun suggestion for your son is a pellet trap, on the website refered to as the 'Cone Pellet Trap'. It holds your paper target and traps the pellets so they don't scatter al over the place. The 'Rocker Pellet Trap' is also a lot of fun for plinking.
Hopefully this helps. Not sure about the availability in North-America but they're great guns and thought to let you know about them. I own a couple of their guns for years now and I'm still satisfied! Fun shooting!