Let him crack on then and try everything else first, as with anything there is a knack to it, cockers are renowned for not wanting to give up retrieves, how many cockers have you trained/owned Kent? As the dog comes in with delivery, go as normal to take the retrieve, once your hands are on the game give the release command, at the same time apply pressure on the dogs paw with your foot, I don't mean stamp on it just a gentle amount and the dog will immediately release game after 2-3 goes of this the dog will release on the command without the pressure on its paw.