I think it is never too late to train a dog. I used a book called Retriever training for the Duck Hunter and it pretty much gives you step by step instructions so it is very oriented to an inexperienced trainer.

It is really just repetition although what you are after is pretty high level stuff that many people never achieve with their dogs. I kept my lab on a lead and released him when a bird was down. On a stalk he would walk at heel and at the shot was off after the duck. I never felt it was worth the time and effort to make him steady to shot. In a river it might be better if the dog is out there quick than retrieving on a line which also might be the case on a duck that still has some life in him.

I would think what the dog already does would be more than adequate for most hunters and for duck hunting I personally would not have much of a problem with the dog as you describe. As far as whistle commands go just use the type of whistle you decide for each command and use it after the command and the dog will make the association. For direction commands, I remember putting dummy's on either side of the dog and then pointing to the one I wanted him to retrieve and whistling for him to fetch it. I also ran him on a lead and would do a quick toot and change direction with a hand signal. After enough times they get the point.

But build the dogs trust in you and he should be easier to train.Trainers are great but they cannot waste to much time on a dog so their methods get results but may be too strong for a more timid dog. I think I would be happy with my good fortune and let the dog go get you some ducks and not worry so much about perfect form, unless you are going to trial or test the dog. If your goal is getting ducks or geese your dog already has enough training to accomplish that.