Minimum two center mass. If that doesnt stop, one to the head, also the pelvic area, the small triangle that runs from your crotch to your hips. Taking a direct hit there releases a large amount of water from your body and causes most people to go into shock. Center mass is always a good starting point though. I would not use all your ammo though, if you miss or minimally wound him he might get angry and you may have to make another controlled shot. Less likely he'll charge if you still have a couple of rounds, plus he may have a buddy nearby.