How tall is your trailer in numbers? If your trailer is shorter than 7'6" there isn't a hope that he'll load into it.
Only the kindest old farts that have been in more trailers than they can count are going to load into anything shorter (this is horses over 16hh).
After that you need to pick a method and stick to it. What has always worked for me is patience and practice and a properly sized horse trailer. My horse was in a trailering accident, so understandably we had "loading issues". He would get so frantic and worked up and start pacing in front of the door to the trailer, trying to run off, etc. I made him halt in front of the trailer, facing into the door and waited. I corrected him if he stepped sideways or back and praised any forward steps. Now I just chuck the lead rope over his neck and stand back so I don't get trampled while he's leaping onto the trailer!! Some horses might benefit from the lunging method mentioned above, but I've always had the most success with just being patient. Budget an hour for loading and then just chill out like you have alllll day. Having a patient attitude is half the battle with loading horses.
They say money doesn't buy happiness -- well happiness doesn't buy horses!