I have an idea.. This is what I would you on my filly training her to stand in the trailer.. does he look nervuous before going into the trailer? With my filly I would let her stand right outside the trailer sniffing it until she felt comfortable enough to jump in.. once I got her in however she would back out as quickly as possible.. so I just started slow and as soon as she got her front feet in I would stop her.. and instead of waiting for her to back out, I would take the enititive and back her out before she thought about it. Sooner or later I would pat her as she would stay in the trailer longer.. then once I could get her to stand with just her 2 feet in there without trying to back out as fast as possible I would get the rest of her in there.. as soon and she got all 4 feet in.. right back out we went.. that way she knew that she could get out.. I would just keep repeating that until she realized she is going to be able to get out.. after a while I did the same thing that I did with just her to feet. Petting her when she was calm and collected and then unloading her..
I hope this can help.. also maybe when he is in the trailer.. when he starts to pull back.. don't tie him up.. let him know that he isnt **stuck** in there and if he wants to he can get out..?
IF IM DOING SOMETHING WRONG CAN SOMEONE PLEASE PLEASE LET ME KNOW.. THESE METHODS HAVE WORKED FOR ALL 4 OF MY HORSES AND SOME THAT I HAVE BEEN TRAINING FOR PEOPLE..