We were recently granted custody of our 14 year old nephew, and I just went through this with him.
There is more than one question to be answered.
Will he be able to cock it, that depends on his build, and physical abilities.
The more powerful the rifle, the more cocking effort it normally takes.
But that being said, where there is a will, there is a way.
If he is slightly built, he may have to adjust how he cocks the rifle.
Also, most break barrel rifles are pellet only.
I agree with pellets instead of bbs, they don't ricochet nearly as forcefully as steel bbs. They are more forgiving.
But MOST break barrel (spring piston guns, and gas ram guns) are hold sensitive. (Search Google for artillery hold)
If you don't hold them they exact way each time, your accuracy will suffer.
The pump up guns and co2 guns are not as hold sensitive.
A break barrel will make you a better shooter overall with regular guns, if you put the consistency into your form.
But the pump ups and co2 are overall easier to shoot.
Personally, I like the .22 caliber air rifles over the .177's
Yes there is a larger selection of .177 pellets at the local Walmart, than there is .22 pellets.
But the high FPS listed on the boxes are rarely what the rifle will actually do in the real world.
And with a heavier pellet, if you wanted to use it for pest control, it just hits things harder..
Right now he is shooting a crosman 1322 with an extended barrel and a shoulder stock, and not very expensive.
It is a pump up, it is easy to pump, accurate, and powerful when pumped up to max.
But you can only out 5 pumps in it for plinking at 10 yards or so..
Good luck, hope we helped, please keep us posted on his progress.