Rhonda,
There are likely multiple factors involved here. How much do you know about this rescue’s past? Is there a basis for his fear aggression? That doesn’t change how you will go about resolving it, but it can help you understand and predict where behaviors will occur. Also, at 5 months, the dog could be going through a normal adolescent fear period. From an evolutionary perspective, at this age, young pups are starting to explore more without their mom around. Building in a heightened sense of fear during this period helped ensure survival of the species as the young dogs start to learn what is safe and what is not.
So how to handle it? I agree that in your house, a crate is a wonderful tool especially if children or strangers are around. If possible, put the crate far enough away that the dog can see or hear the people without being overly stressed by them. Give the dog a stuffed Kong or something else that is very appealing to him so that he starts associating visitors with good things. Do NOT let the visitors approach him or do something that the dog will perceive as threatening. When I started with Lance, this meant he was in my office with the door closed where he could still hear guests but not see them. I would periodically during the visit go give him a chicken jerky strip or something like that. Once he was not bothered by the guests (other than the initial warning alert that there are intruders in the home!!!!), I would leave the door open so he could catch glimpses of the people. Then I advanced to having him on his cot at the far end of the room or laying under my legs. You get the idea – very gradual advancement of difficulty, but only within his comfort level and with the actions of the guests carefully controlled. Top priority is always to keep the guests safe. If it is children who are likely to be noisy and move quickly and unexpectedly, to this day I will keep Lance separated and behind closed doors. It is just too much to expect of him to deal with them.
Outside the home, start with a quiet park or at a time when there are few people there. I would definitely not take him to a dog park or force him to play with other dogs during class. I like the suggestion that you continue to do obedience training (with a high rate of positive reinforcement) while the other pups play. Do not shelter him from scary stuff, but only expose him at a safe distance. “Safe” means that he can see something that might be a concern to him but he is also still able to turn his focus to you and respond to training. Remember that with a change in distraction level, you will need to drop your expectations back to step one. So even if he is 100% on “Sit” at home, when he is in the park with a distraction, return to luring the sit. Then gradually build back up to 100% response. Each time you move closer to the distraction, lower your expectations. Only move closer when he is comfortable at the prior distance. Throughout this process, he is learning to trust you and that you will keep him safe, so don’t break this trust by trying to rush the results.
Definitely watch Robert’s many “Join the lesson” videos to find out how YOUR tension can be going right down the line to your dog. I wouldn’t necessarily rush to a prong collar. The last thing you want is the pup to associate the pain / pinch of the prong with the feared objects: “See – I was right to be scared! I try to drive that thing off and it hurts me!” Also, in Robert’s latest video with the Dutchie-Mal mix, you can see the almost instantaneous change when he takes a fearful reactive dog off the prong and onto a slip lead. He talks about how a prong can make a reactive dog worse.
Consistent exposure is key. That doesn’t necessarily mean every day, though. Things will happen, and if your dog has a particular scare one day, you might want to avoid any triggers for a couple of days before restarting. It can take adrenaline levels over 24 hours to return to base level after a fight / flight response is triggered. If this is related to a fear period, you could very well see a substantial change within weeks. Regardless, keep working on it bit by bit. Your pup is still very young, and much of his confidence will come from seeing you stay calm and unconcerned. Then you will have a long and loving companionship for years to come. Good luck!