I would suggest a gas auto 12 gauge. If he's in FFA he's probably stronger than a town kid the same size, and can handle the weight.
The short stock will make it even more nose-heavy than the usual auto, so you might want to add a half pound of lead to the butt while you're cutting it. It will balance better (change the amount of lead until it does) and will kick less. Add a really good pad, and a shell catcher is nice so he can save his empties rather than toss them at the shooter next to him on the line.
Since he's growing, you might want to buy a Kick-killer pad that goes on with Velcro. They do a good job, and can be moved to another stock when he grows into it.
Since cutting a stock raises the comb and makes it shoot higher, which is what you want a trap gun to do, you shouldn't have to do another thing to it.
