Remember he can’t see the ground behind the trailer, so he’s probably worried that he will just step off into empty space. Try to park the trailer on a grassy hill so the step up and down is very small and easy. Keep playing with it over several days and give treats when he goes in and when he backs out. Keep doing it until he is calm and easy before asking for a bigger step up and down. I think it’s safer learning to back out because if they turn around and go out head first, they will usually make a big leap off. He will eventually trust you and trust that the ground is back there with more time and experience. Good Luck!