Being a bully breed, dog aggression is to be expected as they mature. I don't know if a muzzle would be a good idea or not.. especially for an English Bulldog. Just don't let him around other dogs, don't take him to the dog park, etc. If you have other dogs, I suggest you keep them seperate. I have met many DA English Bulldogs. Bully breeds like to be with their people, not so much other dogs.