First, let the nephew tell you what his recoil tolerance threshold is. Easy to find out: take him to the range and let him tell you what he can handle. Do not poison the well by telling him this one is brutal, this one will break your shoulder, etc. Seen enough douche bags at the range that are teaching others to shoot.
Second, just grab a 6 CM/6.5 CM/7mm-08 and put a Limbsaver on it. Then buy or develop "reduced recoil" ammo for him to shoot for the time being. Then over time he can transition to regular ammo.