I had a dog that was similar to yours, he went red zone around certain dogs but not all, sometimes he just wanted to play.
It sounds like the main thing is that your dog is fearful and lacks confidence and doesnt feel safe. If I were you Id concentrate on building the relationship with him where he feels protected by you, he needs a VERY structered enviroment to feel safe. little things like when your on a walk and you see another dog, make him sit and do some obedience with high value treats, keeping yourself between him and the other dog, when he looks at the other dog, redirect (I suggest teaching him the "watch me", it really helped my dog). I had to spend a lot of time with my dog showing him that I saw the things he was threatened by and was going to be leader. I did have a lot of loose dogs though so when approached I would step in front of him (holding him behind me with the leash because there was no way to do obedience while being rushed and it would not make me look like a leader ) and confronted and chased the other dog away. just little things to show the him that I was going to take the inititive if anything happened. Basic obedience in different enviroments helped him bond to me.
another thing, what if you talked to the owner of the Boston and had a time where maybe you were both in the parking lot, sitting on opposite ends, to where you could work your dog and get him to see that there was no threat, and that you were in charge of the situation? You know, to where you could work with him in more than a hit and run situation of another dog passing?