He needs to be reprimanded when he does this. Get a can and put some coins in it and take it on walks with you. Whenever he starts barking and growling, shake the can and tell him to HUSH!
Someone just recently posted that she got an Altoids tin (the round one, about the size of snuff) and put some pennies in it. She said it is really helping with her barking problem!
Good luck to you. Hope you can nip this behavior in the bud. I can imagine how embarrassing it must be!
