Not being a pro, I'd try force breaking him to retrieve. Shouldn't be too difficult with a dog that is already retrieving. Are you familliar with forced retrieves?

A couple things to remember if you choose to go this route; first, once you start you must finish the program. Second, take your time. Slow, with minimal pressure. 5 minutes a day.

If I were in your shoes I'd wait until the shooting season is over before starting the program, and I'd avoid snipe as best I could. The more mangled snipe you recieve, the more frustrated you'll become, and as hard as you might try the dog will sense this. Keep him on game that end with positive retrieves.

Hope this is a help.

Good luck.