Put him on a lead and don't give him the reward of a retrieve if he doesn't do what you want.

This. I'm not sure how old your dog is, but he's not steady. He's breaking and that can be very dangerous for him.
It's your responsibility to handle your dog, so put the gun down and let your buddies shoot until you get this fixed.
The way I see it, you have two options:
1. Put a lead on him, tie him to a tree with enough slack so that he doesn't choke himself wanting to go. If he moves, whines or breaks...you go pick up the bird. If he sits there quietly, like a good citizen, you let him pick it up.

2. Leave him at home during hunts, and work on this problem at home...AKA, train him to do what you want and fix the problem. Practice the good behavior you want.

I lean towards option #2, but that's me.

It will take time, but I bet he'll figure it out.