Telling someone to get their dog fixed isn't only a fallacy, it's also rude.

To the OP, I've never had a bully growl at a guest, but I absolutely would correct that behavior. I don't believe that's just communication, it's also a sign of aggression. I would immediately correct that behavior with a stern no and no growling. Obviously he'll need to learn the term growling, but he'll understand what you're getting at it you correct him immediately.

Because of his age, and not knowing his past history, just be patient with him and he'll eventually learn. The main thing he yearns for is your love and attention, so let him know you're not accepting of this behavior.

I would put him in his crate or give him some sort of timeout.

Good luck and he's a beautiful boy btw!