Hold
Gloved Hand - start by just getting the dog to "submit" with gloved hand. For bad fighters use just a finger to start with at edge of lip and sometimes having the dog prop up on your leg will do the trick.
Paint roller - introduce with a couple short retrievers
Proofing - False Grab pretending to take the roller but don't
Tapping on the ends to ensure a good hold
Walking Hold- start out by sitting next to him and when he holds, stand up. Now each time you insert the roller do so standing next to him
FETCH
Fetch with roller-
Looking for it - once he is fetching have him start seeking the relief
Taking a step - When he will seek the roller, get him reaching for it by holding it out just enough that he needs a small step to get to it. Continue to move it out some as he reaches. Back him up into position with it still in his mouth.
Lowering the roller down- now start by having him fetch "up" a couple times then start lowering 3-4 inches with each repetition. Once he will fetch the roller with an end touching the ground, swap for the bumper.