Being specific to a particular species of bird, this dog probably had a bad experience retrieving a snipe once.

Two things come to mind; either the dog will relax over time, realizing the snipe are harmless. The bad incident slowly being forgotten. This will take time and patience. Or, force break your dog to retrieve. It might seem a bit strange to force break a naturally retrieving dog, but the training turns the retrieve into some thing he must do, not something he can do. Also, as the same command is used to send him on all retrieves he knows what is expected of him.

How does he do on woodcock?