My dog loves a ball. He won’t perform a down stay when I throw a ball. Should I use an electric collar?

My dog is totally ball crazy. He always walks around the house with it in his mouth and is always ready to fetch. For exercise I take him to the park and let him off leash I hit the tennis ball with a racquet to get some distance. I put him in a down-stay and as soon as I hit the ball he is off after it. Totally ignoring the down-stay. At all other times he will correctly execute the down stay even with loose dogs running around him. I have pulled him back to his down-stay spot, after tricking him by faking hitting the ball because I thought by the time he gets back from fetching a football length he has totally forgotten the down-stay command, by the back of his neck and with a little ear. And even right after I put him back and hit the ball he is off again. The only thing that worked so far is putting my foot on top of him. Should I continue with the foot or is there something else I can do? Is it shock collar time? One more thing. Is the 3 second rule credible? Which is if you don't correct you're dog in three seconds after an incorrect behavior you might as well not correct. Because he won't know what he did wrong?

Thanks again,
Chris