Instead, load him up and the second he starts to back out, you take charge and back him out. Once he's out of the trailer keep backing him all the way to the property line or lunge him for a little bit and then load him again. The second he starts to back out, do it all over again. At some point he will think, "boy, it sure is a lot easier standing in this trailer than backing out and doing all that work!"
Joel I would disagree with your advice simply because it skips steps.
OP should not be loading horse completely until he/she has 100% control of the feet at all times. The horse shouldn't be deciding to back off the trailer; the handler is the one that is deciding when and where to move the feet. So that by the time you DO get to the point of loading the horse fully, the horse isn't going to back out on their own because you've prepared them for that moment. They are going to wait for your next cue.
Which strangely, my next cue WOULD be to back them off.
If you also take a moment to read the link I posted, the method that's worked for me is one foot on, one foot off. Repeat 5,000 times. Then two feet on, two feet off. Repeat 10,000 times. Then three feet on; three feet off. And finally, when you load all 4 feet, you are going to ask them (before they even think about it) to take all 4 feet off.
It's amazing when you keep asking them to go on and off a million times that they just CRAVE staying ON the trailer and will gladly stand in there and wait for you, and hope you don't make them back off again!
Just as I don't want the horse to decide to back off the trailer (until I ask them to) I also do NOT want them to load themselves until I ask them to.
Again, it actually has very little to do with the trailer itself. It's simply an "obstacle" that you are working with and putting your horse's feet on and off it. Basic ground work with foot control!