HI, I'm Dr Steve. I would like to give you some ideas that may help Louie's fear. First, as you realize, fear aggression is a very difficult condition to treat and many dogs will improve, but they still have the potential to bite, no matter how well they seem to be doing or how long they have gone without acting aggressive. These dogs unfortunately can not be trusted. This is not to say they can't be god pets and can not give pleasure to their owners, but they simply have to be kept in situations where they are not around people that may frighten them and cause enough fear to make them aggressive.
With that said, you are going in the right direction with exposing him to many different people, of course he needs to be leashed and completely under your control when doing this for his protection and other people's safety.
ALl dogs are different depending on the level of fear they are feeling. In my experience it is better in most cases to expose him to strangers, but try to do it one person at a time. If he is on a walk with many people, it may cause too much fear and he may not be abel to understand that he is not being harmed. It is like overloading him with scary experiences.
Taking him out and trying to limit his interaction with one or two people at a time and then giving him praise an a treat when he doesn't act aggressive may help.
Having people pass by and not stopping to interact with him may be best. Trying to give him short, exposures and talking to him as the person goes by, then when he doesn't misbehave, give him praise and a treat. The idea here is to create an opportunity for him to be good and then praise him. If he does growl or act aggressive. give a firm tug on the leash and a firm "NO", then move on to the next opportunity. As he starts to do well with one person and short exposures, you could move to two people or a small group, but again pass by them and when you are away from them ( to make him continue to feel safe) give him praise and treats.
The key is to give him the chance to be good so he realizes the reward he will get. It is also important to give him a "NO" when he is not good so he can learn from positive as well as negative reinforcement. Hopefully with time he will learn how to act, but again he may never fully trust people and that is why he may still bite under the right conditions. He may learn to be good on the leash, but not be able to be off leash around strangers.
Also be careful even with household members, as well as yourself. If he is being ok with you, there is still the potential that if you move too suddenly or do something that doesn't seem frightening to you, but is frightening to him , he may react and bite. Just something to keep in the back of your mind.
There are also medicines that can be used to help. These are antianxiety medicines that need to be given once a day. They help reduce anxiety, not eliminate it. Some dogs become less frightened when in the medicine and become a little easier to train. I hvae used Clomicalm and Reconcile. These are not meant to be used forever, but for 4-6 months to help a dog learn to not be alarmed by stressful situations. They reduce the anxiety to some degree so people nd events may not seem quite as frightening as they wold without the medicines. These are available from your vet. I have had people try behavior modification like I have described and if no improvement , then they have added the medicine along with behavior modification.
I commend you for taaking the time, effort and responsibility to help him cope with his fear and hope this has helped. If it has would you please rate my answer. I will be back online later this evening (US time) to see if you have more questions. THanks for the question and god luck