The youth model 870 is about the only gun I ever use for birds. It was given to me when I was 10 years old and though I'm now beyond youth age, I find the short barrel much easier to control given my short stature. It's slaughtered it's fair share of ducks, geese, doves and turkeys alike.

As for suggestions, ask ten different people and you'll get ten different answers. I use a Primos Jellyhead choke for turkeys and the pattern stays tight to about 30 yards. At 35 yards, the shot scatters a bit but would definitely still kill a bird. 40 yards, less confident, but could probably still kill a bird if it's a clear shot across a field. I definitely wouldn't even consider shooting any further than that with it. I use either Winchester Double X or Kent Ultimate Diamond Shot, 3" shells 5 shot. Both pattern relatively the same out of my gun. I've used several different guns to kill turkeys, but that combo is my only go-to nowadays. The only time it's ever failed me were for reasons that were my own fault, never the gun.

Best of luck to you and your son this season. It's a great gun, he'll definitely slaughter some birds with it.