A single shot from Harrington and Richardson, available at your local Wal-Mart. They prolly won't have 'em in stock, but they have a special order catalog that don't cost any extra. It will be around $112 plus tax and background check. Good luck!
i've only shot trap with my clays/hunting/blasting/all-purpose mossberg 835. pretty much competes with the remington 870. just a good, versatile pump gun, coming in around $300 give or take. i'll probably get a remington 11-87 some day, but i think unless you're getting very competetive it's unlikely you'd need a dedicated gun for clays. maybe i'm just nuts. function defeats form in my book
... well most of the time. when i can combine the two that's great. and it doesn't apply for women.
How big is your son? If he's grown or nearly so, a regular full size pump shotgun like an 870 or one of the semis is best. Make and action type is not so important as fit and felt recoil,AKA kick.
Start him off with the lightest loads you can find.