Don't overlook a 20 gauge. Unless he is a dedicated waterfowler, a 20 will handle all upland game, skeet and sporting clays and neither overburden him with it's bulk or kick the snot out of him.
Don't want the lad to develop a flinch, do we?
Unless he is a very experienced gun handler, a pump (or even an over/under) would be a better choice than an autoloader for teens first shotgun. And unless he has adult length arms, consider a youth model or shorten the LOP. You can always get another buttstock later on when he grows some more.
