Get him a 12 gauge semi auto used,  Beretta 302 or newer version or Remington 1100.  But get a "Trap" gun,the stock is different and it will hurt him less. Also make sure he's holding it right.  When I started I was shooting a Remington 870 competition trap and it was beating my face up and hurt my shoulder a little.  Finally learned how to hold it and anything I've shot since doesn't hurt at all.