As is usually the case, this is a problem with the dog pack hierarchy. Dog's brains are wired to be part of a pack and to assign each dog (or person) a level on the hierarchy of the pack. Obviously, the pack leader is the most respected. Your dog views you as below him on the hierarchy. This is why he feels it's ok to growl at you. A dog will never challenge who they see as pack leader. You need to become pack leader to control this. Bulldogs are tough because dominance is what they live for. I suggest you read up and maybe get some help because this could escalate.