Another Dog Question

[meaux fishing]

Ive been working with my lab, who I inherited when I met my wife, for a year or two. I can get him to stay 95% of the time and he will retrive dummys and dead birds in the back yard all day. I took him out for his first time hunting ever during teal season, and he had no trouble marking the bird, but he would go out to the bird, pick it up, walk a little ways back, then drop it and just stand there. Any ideas how to make him complete the retrieve in the field? Maybe he just doesnt have enough real world experience? He is around 4 years old so Im pretty happy with the progress he has made so far from just being a spoiled house dog.