Common theme here from trapshooters: get him a gas powered semi-auto. They really reduce recoil. How big is he? He can likely handle the weight of a 12 gauge. I bought my son a Berretta 303 20 guage, but within a year he went to a 12 guage 391 Beretta. Remington 1187 is also very good. Don’t buy a recoil operated auto like a Bennelli - they kick like a mule.
Also don’t buy the cheap Walmart Federal type shells. They are hot loads and kick too much. Get some 7/8 oz. or 1 oz. loads. 
;