First off, I own a 410/45 Colt revolver, and am very satisfied with it's performance for my purpose. So this isn't a bash of that platform. However, the 410 bore in a handgun will not make up for poor marksmanship.Period!
I'd say If it is to be his gun, it should be his choice. It should be which ever he feels fits him best. Maybe the proper fit, and feel of a handgun, no matter what chambering, could make more difference than shot or bullet.
Cheapshooter's rules of gun ownership #1: NEVER SELL OR TRADE ANYTHING!