What is the next step in curing dog aggression?

My American Bulldog puppy started showing aggression at 14 weeks, on walks he would just stop and sit when anyone passed him. Then it progressed to growling and eventually barking and growling at around 5 months. I started him in a puppy class and he is now down to only growling if a dog or person gets within 5 feet. We have established he is fear aggressive because he puts his ears back and tries to hide behind me while growling at the trainer. Now when we take walks he can pass people without sitting down and staring or growling and barking, but he tries to speed up and get away as soon as we pass them. So my question is, now that it is at least manageable, I am confident I can turn it around but I don't know what the next step is. I live in a small town so park, residential areas, downtown are basically barren of people. I walk him every day for an hour and I am surprised if I see 2 people in one walk. I think this contributed to his fear problems, but what do I do now? I have introduced him to everyone I know and he's fine with them, and I'm still scared if I ask a stranger to give him a treat he will bite. What now?