puppies bite, it's normal. wackycat, slapping his muzzle probably isn't the best way - the dog can interpret that as a type of play, especially a big dog with a high pain threshhold that likes rough play. if he's very play motivated, just withdraw from playing. if he's more normal, i've had good luck with catching the muzzle and holding it trapped for long enough that he realizes he doesn't like that.
most pups pretty much grow out of it on their own, but you do want to give consistent negative feedback to help it do so - especially since your dog looks like it might have some pitbull or something like that in it.