im 19yrs old, and am DEFINITELY not a big person, 145 soakin wet, and i started hunting at about 11-12yrs of age, with a 20ga. Never had a problem with it as far as recoil, and i have always been small in stature for my age, especially at your son's age.

Have him use a 20ga, its a wiser move, plus a 20 is a more functional gun for other game, not knocking the .410 but it just doesnt have the versatility of the 20ga, plus you can get turkey shells and choles for the 20ga, you wont find any of that for the .410.

my $.02

Mat