Cutting the stock on his .410 is an option but the gun will forever be "youth" sized unless you can replace the stock. If you have a fairly low cost single shot like the NEF I would cut it for him. It is fairly easy to get parts for them so later you could replace the cut stock with a full size. I would also at least talk to someone that has done this before, you do not want to change the angle of the butt. There is a book that you can pickup at the local WalMart for about $14 that will show you how to cut the stock properly, click the link below to view it. Good luck.
[link]http://www.walmart.com/catalog/product.gsp?product_id=235420&cat=22191&type=3&dep t=3920&path=0%3A3920%3A18888%3A22191[/link]