It you haven't gotten a vet out there, get one out immediately
! Whenever any of my animals or my lease horse act off, I get one out as quickly as possible! He may have some sort of illness that was triggered by the move, and the new farm. My friend's horse was acting super off after they just switched her hay, and it turned out she had some sort of genetic liver disease (cannot remember which). The simple switch was the catalyst that led to her having to be put down.
Or it could be that something about the new farm triggered memories of past abuses (more horses, people, activity, etc) and he reacted to that?