Is your trailer aluminium or shiny? He might be seeing his reflection, and doesn't understand what it is. I would suggest tying a hay bag in front of him, but remembered you don't feed hay.
No my trailer is just a plain one. I was thinking tho of hanging a hay net. I do feed hay occasionly, just not regular.
Actually, been thinking I might put the trailer in the arena and hand his feed bucket on it and lead him up to it that way. After some just walking around it for a few days.
Becky