A lot depends on what fits him. If he is small framed and light I's opt for one of the 20 guage youth guns. Ny one daughter fits that description and she used her 870 youth 20 gauge for turkeys, pheasants and waterfowl successfully. Her setup will thump a turkey dead out to 35+ yards.

Otherwise get him a 12 gauge. It's just a bit more versitile and easier to find ammo choices.