The hardest part, is that if you outright accuse him, he may get hostile or defensive. Maybe you could bring it up that a co-worker saw they were having some problems as they were leaving and thought they could use some suggestions of how to control the dogs behavior. Obviously if you see it yourself, or if it seems to be continuing you may need to take more drastic measures. Obviously punching a dog is never okay, but if there is a less confrontational way to bring it up, he may be more receptive.