One step at a time. First on the ramp and always let him step back and off or you will scare him more.
Never lock a scared or tense horse in the trailer.
If he steps twice on the ramp make him go down clamly yourself before he decides to.
Use a long rope and no pushing a whipping from behind.
Prepare yourself to the fact that it might take a while.
I also reccomend some Parelli videos on YouTube.
My only advice is: be patient and do not burn the steps!
Most of the time, horses are less afraid of a big horse trailer, they feel less shut up and cramped. The presence of another horse in the trailer can really help :)
Patience is the solution to every problem with horses... Work 5 minutes on this exercice everyday, not more than 15 minutes!
Time, lots of carrots, and a great ability at keeping calm. Last resort: have another horse get in the trailer before yours so he sees it's not such a big deal.