Does he go on regular walks? Did anything in particular ever spook him?
I'm asking because, after I moved, my dogs had a huge yard available, and I didn't walk them on a daily basis for a while. So, between being in a totally new neighborhood w/new-everything (people, buildings, other dogs, smells... you name it) and having some sort of weird "fear-phase" ( I
think that's what it was, anyway
) my older Pit Bull did a sort of strange recon-stealth walk and literally crept around like he was in enemy territory for a while. And he was always in a hurry to get back home and into familiar surroundings...
I kept our walks short and brought lots of treats in the beginning. I also walked him around when it was fairly quiet and when there weren't too many people outside. Walks are slowly getting better... and longer.