I'd shoot 4s, 5s or 6s, preferably in Hevishot. I don't know if Environmetal makes Hevishot in 16 gauge or not, but if they do it is a 'no-brainer' to use that stuff. It is the very best stuff I've ever seen for killing gobblers stone dead.
2s are a poor choice for turkeys, as you don't get nearly as dense a pattern, and as has been mentioned, you're not shooting the bird, you're shooting his head and neck. If your son isn't real experienced, tell him the best aiming point on a tom is right where his wattles are, at the junction of feathers and bare skin. Lots of inexperienced turkey hunters aim for the head, but that is a mistake, as you waste a large part of your pattern going harmlessly over the bird's head. Aim for the wattles, and you'll have pellets in lots of lethal places. Be sure to shoot when his neck is extended, not tucked down in strut position.