Just my $.02 Instead of going straight to the stem collar, start working on obedience and denials. He is so ready to go he cant stand it because he knows every retrieve is his. Show him he doesn't get every retrieve, in training and hunting. If he whines never let him pick one up. Only send him if he's quite. Once he understands this he'll calm down.
A dog has to know why he is getting stemmed to correct it. Just because you say NO then stem him he might think he's getting trouble for sitting wrong, looking in the wrong direction, shivering, etc.... I love stem collars, but be careful!!! I don't use one with my DuckHill Kennels dog but I have one on my police dog. I rarely use it, but in a high stress situation it might come in handy. I don't want to ruffle any feathers. There are a lot of opinions out there I just wanted to join in a conversation. There is nothing like hunting with a dog!
this is also good advice. we here, who offer free advice, recognize that all dogs are different. one size does not fit all.
the absolute biggest mistake that duck hunters will make is: we train our dogs hard in training conditions, but refuse to leave our guns home when the season opens, and continue the training in real field conditions. big mistake. as the above post says........there is no way to replicate the real life conditions of a duck hunt in your back yard. the training has to continue in the field. but we like killing ducks. that's what we do! :)