Yes, you stop him halfway through. Maybe you can have him roll close to you so you can stop him. Give him a treat and a tummy rub while you keep him on his back.
You can put him in a Down, then lure his head over his shoulder with a treat. Put your hand on his tummy right away so he doesn't go all the way over.
My trick book says to practice this with Roll Over, so he understands the difference.
I'm jealous you have such a smartie. It took Thor and I soooooo long to get Roll Over, that I don't want to push it for a while. He's still not 100% on it. Also, he is not comfortable on his back (he will never sleep on his back, for example), so I think Bang! will be very challenging for him.