My 10 y/o son used a 357 Max rifle last year to harvest his first deer. The handload was light, a 200gr LHP at 1160fps. This year he is able to shoot a 180gr Fury JFP at 2000fps. If your nephew is on the small side or recoil sensitive I would lean towards a jacketed 140gr or 158gr bullet designed for and shot at 38 Spec or 357 Mag velocities. Keep in mind the velocity will be faster from the longer barrel and could push pistol bullets beyond their design capability. The 357 Max gets my vote.
