As mentioned, the free state version of the M&P Sport would be a good rifle for a boy. M&P 15 Sport
Hold up... it appears that there is a new and improved M&P 15 Sport in town... S&W M&P 15 Sport II
Looks like they added a forward assist and a dust cover - but still used the 1:9 twist like the later models.
After a few years and thousands of rounds, I've never needed the forward assist on my S&W MP15 Sport because the rifle always (always) works, and I see no need for the dust cover - even for my truck gun working/shooting in the field. YMMV - if you actually plan to roll in the dirt, it might a great feature - if you remember to close it between shots.
I prefer the original 1:8 5R barrel that they put on the first generation M&P Sport models that I have.
As far as how they work, I cannot say enough good things about the perfect reliability of the two that I own. One for the truck, the other at home for the kids/grandkids to blaze away with. Ten to twenty (30) round mags are typical for an afternoon at the tank.
I'm a "one is none and two is one" kind of guy without deep pockets. So, I prefer to own two $600 super reliable AR's than one...... There are many super reliable, inexpensive AR's out there. Some may serve your needs waaaay, waaaaay better than mine.
MY point is only to share my own hands on experience.