Gun shop guy is full of BS. Unless this kid has experienced at least moderate recoil, a typical 2 3/4" heavy load will stomp his skinny self him flat and probably scare the pee out of him.

A 20 ga., 3" turkey loads and a full choke, or "special" turkey choke are a great combination for gobbler getting. Recoil of a 20 ga. 3" turkey load is not timid however. Heck of a lot less that a typical 12 ga. for sure, but still a pretty good whop for an inexperienced shooter. You got a .410 he can practice with? This will get him used to a decent bit of recoil and might give confidence in using a heavier load.

If practical for you to do so, I'd be sure to do the best I could as far as fitting his length of pull. If you are not sure if the kid will get into hunting, you might consider an inexpensive (relatively??) single shot and have the stock cut down to fit him.

Anyway, have fun and enjoy the experience.