I have a fear aggressive Pit Bull/ American Bulldog?

My bully breed dog is 6 months old and has had moderate socialization, but obviously not enough because he growls at strangers and strange dogs. He is fine with my family and a few friends and their dogs, and I also have three other dogs in which he gets along great with. However, I live in a small town, and people to introduce him to are limited. I took him to the doggy park at 12 weeks and he was fine and played with the other dogs, but then they kicked us out because he was supposed to be at least 16 weeks to be in there, then when I came back he hadn't had dog socialization for that whole month and he was growling at everything. And he even went to a church banquet with me and was fine, being pet by kids and everyone. But like i said, things to do where I am are limited, and when I take him for walks around the neighborhood or park, we rarely even see people. I was surprised to see 2 people on my last walk. And I walk for 30-45 minutes and several blocks in a residential area. When he sees someone he just sits down and looks at them and back to me, and if they get within 5 feet or so, he hides behind me and the growling begins. So, I recently started a dog cl and at the second cl he is fine unless someone gets within 5 feet of him, then he growls. The trainer says she doesn't know if he can ever be a normal dog. Today was the second cl and she approached him and after 5 minutes of coaxing him he finally took a treat from her and she said 'good boy' then she turned around and he barked and lunged at her. It was scary for me and the others in the cl were stunned. I want to know if he could ever be a normal dog and accept strangers, both humans and dogs, or if I'll have to keep him away from strangers for risk of biting, or even worse... put him down. Can someone help me?