Unless the factory choke tube is stamped "Modified Steel Shot", steel hot constriction for "Factory Mod" will likely be equivalent to full choke.
Improved cylinder choke with smaller shot size=less restriction=less recoil=less flinch. I even tried to purchase the cylinder choke for my son's shotgun several years ago.
For initial success for beginner, suggest 2 3/4 #4 shot @ around 1,200 fps at shots between 25-35 yards. Kid can get an fair idea of how far this is by marker decoys, clearly seeing duck's eye, pre-season helium balloon on kite string out to 25-30 yards, etc.
Some adults have had excellent success with 20-gauge using #6 shot over decoys probably no more than 25-30 yard shots.
A no-no is getting 3-inch mag 1,500+ fps shells on the market for a first-time beginner.