When Bella was younger I would tap lightly with one finger ( just to get her attention) on her nose and say in a low drawn out voice "nooooooooooooooooooooooooo" I wasn't mean or mad i just wanted her to know that behavior would not be tolerated.
You could also practice havng a family member pet the pup in your presence and if he didnt growl then give him a treat.
Now all i have to do is whisper noooooooooooooo and she looks at me like "uh oh i am in trouble"
Good luck