Rotties can be "chatty". What sounds like a growl can be "talking". Now, I am not saying that your puppy isn't growling - just that it could be something else.
I am not going to say whether your method of stopping his growling at you was right or wrong. Generally speaking, staring an unfamiliar, adult dog can be dangerous. But you are dealing with a young puppy. He doesn't need to challenge you for the alpha position - you ARE the alpha.
Thank you for pointing your daughter's BF in the right direction. Classes will be invaluable.