u/Fantastic-Purposes

I adopted a rescue dog a few months ago, and overall he's doing great at home. But whenever we see another dog on our walks, he turns into a huge anxious dog. He freezes, pants, and sometimes whimpers or tries to hide behind me.
I really want to start some gentle rescue dog training to help him build confidence, but I'm not sure where to begin. I don't want to push him too far past his comfort zone.
For those who have worked with a rescue dog anxious around other dogs, what actually helped you in the beginning?