I think the M&P15-22 is an excellent choice but have him look at the Colt/Walther M4 .22LR too. He'll know which one he wants.
I narrowed it down to these two and chose the Colt because it looks more like a real M4, has metal receivers and look alike (but often non-functioning) items like the forward assist, BHO, dust cover, flash hider, etc. It's also the same weight as the M4 and has a 30rd mag instead of the S&W 25rd. Disadvantages are: harder to break down/clean, doesn't use an AR FCG, a pencil barrel, foreign made (Germany) and not as popular.
The S&W is much more popular, lighter and operates much like a real AR though doesn't have the same look and feel. It's usually less expensive and is made in the USA. Most of its disadvantages (at least in the basic model) to the Colt are completely missing items like the forward assist, dust cover, flash hider, etc. and the 25rd magazine.
I'm real happy with the Colt and got it for $450 in town, a decent price. That same day I could have gotten the Smith for $400. The Colt is very reliable and so is the Smith (I hear) now that both have got the kinks worked out of early production. I suspect I'd like the S&W just as well for a shooter, but just liked the look of the Colt and having the Colt rollmark even though it's actually a Walther.
The one pound lighter weight of the Smith might be an advantage for a boy if he's very young.