I would be tempted to go back to the training classes, but go slowly and at his pace. I had a very shy little dog and took him to classes and he refused to put his feet on the floor!! Fine, I sat with him on my knee for a couple of weeks and let him watch. Then I carried him round the hall for weeks putting him down only for sits, downs etc. Everyone thought I was crazy but I couldn't have cared less. Eventually he was brave enough to walk round the hall, do the exercises and socialise with the other dogs, although it was a long road. The work I put in at the beginning paid off and I was always able to walk him off lead and know he was sound.
The popping other dogs while he is on the lead could be because you have removed his option of flight. Do you have any secure areas where he couldn't bolt onto a road if he got frightened? If you have then it might be an idea to try him there with a few dogs that he knows to see what he is like.
You could of course
just send him to me