Depending on what your expectations are, and how much interest this dog has in retrieving, you should be able to at least develop him to what would be considered a "started dog" level. If he's had basic obedience, conditioned to gunfire, and picks up and holds feathered game without hesitation, then you at least have some building blocks from which a retriever can be developed. I do know there have been some top field trial performers that didn't get their start until they were over a year old, but depending on your dog's interest level (or lack thereof), you may be better off with trying to start a new dog than trying to break an older one. It really boils down to the dog, and what you have to work with.
Feel free to PM me if you have any more questions.