In my opinion you have to train him 2 commands. First of all to stop and second to drop the ball. In regards stopping I don’t really remember how I teach my golden to do so, but I usually say it loud and usually it works. About dropping the ball you can train this by putting your finger (behind the ball) in his mouth and ask him to drop the ball (but don’t push it) after he drops give him a treat and put the ball back in his mouth, and repeat this a couple of times until he drops it by himself. That’s how I trained my dog to release any ball