That 20 guage will do just fine, I started my girl out with a 410 when she was very young and we never had a miss. Call them in as close as possible and use a stick in the ground beyond your decoys to mark the maximum distance you are comfortable shooting, after your son has practiced and patterned his gun. You can print turkeys head targets from the internet, or buy some. Make sure you're getting a good number of pellets concentrated in the head, at whatever range your shotgun shoots best. Your maximum effective range will depend on the gun and ammo used. You may find that your gun patterns best at say 15 yds this year and kill your bird within that range. Then you may try other ammo and find your range extended to 25-30 yds for your next hunt. Just keep experimenting with ammo as often as you can, and when you are satisfied with a load stick with it.