As Fred pointed out, the key will be prey drive. You mentioned that you would simply use him for retrieving (as opposed to upland hunting). If he's sitting in a California blind all day, and retrieving only a handful of birds, he would probably be fine. I think Fred is also correct in that a lot of old dogs that haven't been in training their whole lives never learned how to learn, if that makes sense (much like humans who never learned how to learn), have trouble learning new skills later in life. But your dog was highly trained, so learning a new game may not present a problem if he loves birds. So get that wing-clipped pigeon out, and see how he responds.
I would also recommend that you get involved in one of the training clubs. In San Diego, there's the San Diego Sporting Dog Club (contact me for more details). In the Inland Empire there's the Inland Valley Retriever Club. You'll meet a lot of good people who are very willing to help you as you train your dog.