This is where force fetch and coller conditioning come in but he might be a little young for that. Here are a couple of tips always have a check cord on the dog you have to be able to correct on the retrieves as well. Use a hallway on the retrieves. He can't get away from you there and it teaches him that retrieves are the same as any command he might get. Finally the old going away trick. When the pup picks up the bumper call him and start walking away he will probably follow and come to you. In any event as he comes to you on any of the above give him lots of praise he'll fiqure it out.