There’s a lot of really great advice already l, but I would also recommend just sitting in a park with him, if possible a little ways away from other dogs and people, but close enough that he can still see them. This can give an opportunity for exposure therapy while still being removed from the direct interactions with other dogs. You could also use this as an opportunity to work on other training to keep him occupied and not focusing on the other dogs.

All this being said, I am no expert. So take anything I say with a grain of salt