Recoil is primarily a function of :
the amount of shot in the shell in ounces
the velocity of the shell
the weight of the gun
Decreasing the load in the shell ( go to 3/4 oz if you can find it ).
Buy a low velocity shell - like 1150 fps
Add some weight to the gun, if he can handle it ( keep it balanced between his hands ).
Or do all three .......
Or see if you can find a 28ga ..... / or consider a gas operated semi-auto in 12 or 20ga ( Beretta 390, 391 / Remington 1100 ) etc.....
Forget the pistol grip, and all that nonsense, none of that will make any difference.