You should be handling him a lot on lead and walking him over a sheet of plywood on the ground so he gets the idea that just because the sound and feel changes does not mean he is going to die.  Do not make it a big deal.  Windows down.  You can have him follow you in or you can train him to walk in on his own.   Makes me nervous because I don't know how much experience you have with horses.  An enclosed trailer can be a dangerous place for you and a thousand pounds of scared and stupid.  You really need to build trust first and that takes time.  I watched a couple with 50 years experience between them show me how they learned to load horses with this really neat trick.  They tied a long rope around the horses barrel and ran it up through the chin strap on the halter.   They then took the end of the rope and wrapped it around a pole in the  trailer to put tension on it and one tapped the rear with a whip.  The horse reared, the rope caused her to be off balance and when her feet went out from under her she came down with a sickening thud on a rock that was in the driveway.  The only good thing was that she died quickly.  Patience, respect, and trust are key.  Do not whip or try to force him.  No real teacher like finding someone who has done it who is willing to work with you.  Best of luck.  dave