It doesn't sound like he's scared of trailer (that was an issue with my horse). Sounds like disrespect. What my trainer did he moved my horse between him and the entrance to the trailer back and forth: move right turn, move left, turn, etc. Basically kept her moving non-stop, when she was ready to stop he pointed to trailer as the only place to stand still and relax. If she refused, she was forced to move again. It worked.