I have a lovely Cavalier King Charles Spaniel, pics on profile for drooling purposes.
We took him to puppy training classes and made sure when he was younger that we socialised him really well. As a result he was perfectly happy meeting all sorts of dogs, pretty much fearless and not worried even by big 'scary' dogs. Brilliant.
Then next door neighbour got a rescue dog. Dog is lovely, and doing really well, but at first he was a bit temperamental. Neighbour had idea to take both out together so they could 'meet' rather than just sniff under the fence.
On walk with neighbour and dog, my dog recklessly attempted to steal other dog's ball from under his nose. He got bitten on the face, not badly, but drew blood.
Since then, almost a year, he is scared of all other dogs. When one approaches he freezes until they go past and if there is a big one where we are headed he won't move.
He goes for a run everyday and meets at least one other dog usually more, so he is having plenty of contact, not being sheltered away or anything. Other than keep doing this, is there anything else I can do?