Have you ever tried clicker training? It's surprisingly easy to get a dog to perform tasks he previously had no interest in. My guy will now pick up all sorts of things he would never have in the past. I taught him a generic "pick it up", and he will pick up the object I indicate on command. He never was interested in balls either, but now will pick them up and bring them back if I ask him to do so.
When I said to quit while they still want more, I meant while they are still really into it and having fun. Builds anticipation for next time, so quit before you want to, when he is having the most fun, rather than taking a chance of boring him.