I'm not sure how old your dog is, but what I did was work up to giving the dogs commands while he was running after the ball. First, I threw the ball and required the dog to sit until I told him to get the ball. We added downs, speak, "around" (circling around me), look at me. Once he got used to having to listen to me, it became easier to give commands after he was sent after the ball. Maybe start with throwing the ball, giving a command, then sending him to fetch, As soon as he's about 3 feet away, give another command (I actually also use "stop" to mean, literally "stop running"). He will get there!
Xan Varik von Fernheim (born 4/3/13)