You dont want a bull bred dog to be 99% some of the time, you want a 100% all of the time when it comes to aggressive behaviour ontoward his master(s) you and your wife. Some sound advice here (especially Moll's), take on board and try and eradicate the behavioural problem ASAP, plain and simple he is being dominant it needs to end before he will follow through from his growling at you(s). Just my opinion, not a personal dig. Goodluck.