Thanks, that does look like a good read, all the acronyms make it a bit harder to get to grips with, but I'm giving it a go.I would agree with what @JudyN suggests above. He will have an invisible radius of space around him where he feels secure (called flight distance; anything within that radius is a threat that his first choice is to flee) . Find out what that is and keep him far enough away from other dogs that he is relaxed. Reward his calm behaviour.
If he reacts like this or even at all, you are too close. Gradually, over weeks and months, not days, work on reducing the distance. But - be aware that if your dog has had a stressful episode the stress hormone can stay in the body for up to 48 hours so a distance he was comfortable with the day before might be too close that day. So the safe distance can change, watch his body language. For that reason also you might want to avoid all triggers for at least 48 hours before you start working on this so he has an empty stress tank.
You might find this interesting - it's quite in depth but a good read.
Start Here - Care for Reactive Dogs