first... what kind of experience does your boy have w/ rifles? is he recoil sensitive? what kind of terrain are we talking about hunting, here?
i used to think the mossberg was a great value in new rifles, but i keep seeing negative reports on it like the one in the link. definitely a caveat emptor situation:
http://www.handloadersbench.com/forum16/1413.html
the rifle doesn't matter so much as long as you stick w/ known quantities like remington, winchester, marlin, ruger, winchester... what does matter is sighting options.
a marlin 336 is a fantastic choice for thick-n'-nasty hunting. if you are going to scope the rifle, then brand of glass does matter, and if you go w/ a bolt action gun, you kind of need to scope it.
good used bolt guns are going to run $325 on up, and things like scopes can add significantly to the price. a good used marlin 336 is going to be in the $250 range.
what to look for: check out the crown of the rifle. a magnifying glass helps, but not necessary. there is a small radius at the bore. check that radius out to make sure it is smooth - no nicks or burrs. if you find one w/ a damaged crown, use that as a negotiating tool - to have a 'smith re-crown a barrel will cost about $50.
look at the face of the bolt. there will likely be some discoloration, even grooves in it, and that is ok. it is not ok if it is heavily pitted.
work the action and make sure there are no hang ups. if it doesn't operate smoothly in the store w/ no ammo, then it surely will cause headaches while trying to feed ammo.
the bluing at the end of the barrel can give an indication about how much the gun has been fired. not that it is a big deal, but just so somebody tells you the gun was never fired, or only fired 50 times or whatever. if the bluing is mostly gone at the muzzle end, it has been fired a lot. like i said, not a big deal, but sort of a 'lie detector'.
the 270 is a good cartridge, but it does have a lot of recoil to kids. i didn't see anything about age, or size of your boy, but also consider (in order of increasing recoil): 260, 7-08, 25-06. the 308, 30-06, and 270 are good ones too, but you may run into recoil issues for a new shooter.