We adopted a 4 yr old cairn 4 months ago.?

He's and ex show/stud dog, had only ever known the show ring, back of a car or a concrete kennel and was afraid of everything at first. We've gradually built up his confidence and he had become a really happy little dog (although still a little bit nervous occasionally) and had just got that we were able to let him off his lead when we went on walks and know that he would come back - he loved it!!. We live on a narrow boat so its quite safe. 4 days ago he suddenly started having to be persuaded to go out for his walks, refuses to walk once he gets out (just wants to get back to the boat) and he's become absolutely terrified of cars all over again. There aren't many driving about around the marina but there is a main road running alongside of it and he appears to jump at every distant car noise. We took him out of the marina for his walk today (a car ride away) thinking he might be paranoid about the marina but he was just as bad, he was frightened of the other dogs as well as the few cars that were there. He just freezes and then tries to dash off. We couldn't possibly let him off the lead now. Anybody offer any help?? Please, please, pretty please??????