Why is my American Bulldog growling at my boyfriend?

My boyfriend and I moved to Colorado from Ohio about a year ago, and we adopted a 2 year old American Bulldog from his previous owner and he loved us both. Shortly after we got him he had to undergo serious surgery to save his life, and it seems like ever since then he changed a bit. He growled at my one friend Annie when she pet him and she wouldn't stop and kept ignoring him and pet him even more and he started to show teeth and she leaned in for a kiss and of course he bit her. She immediately told me I need to put him down, but I was definitely not going to do that because he bit because she was ignoring his warnings. But after the bite my boyfriend took him into the kitchen and corrected him (no hitting, he yelled a bit which I disagreed with) but ever since aj disciplined him he constantly growls at Aj. Even if Aj comes over to pet him he growls and sometimes shows teeth. He never does this to me, I'm wondering if he was scared of Aj yelling. I want to fix this because now he is starting to growl at guys who are friends of ours who just pet him. Please help! Also he does not like other dogs (as american bulldogs usually don't) and we have trained on that and it has gotten better. But I can't have our dog growling at Aj because we all live together. Please help!