The first thing that jumps out at me is that at first he loaded. Then, he stopped wanting to go in.
I have seen this the most when people drive too roughly with a horse in a straight load. When a horse is in a slant load, they can hold their balance much better because their right front foot is out there to balance on. If you haul horses in an open stock trailer, they will stand that way any time they are tied with their heads at the front. If they are untied, they will usually haul standing backwards.
When a horse is hauled in a straight load, you MUST drive around corners and curves MUCH slower and more carefully that when pulling a stock or slant trailer. If the divider goes all of the way to the floor, it should be cut off, replaced with a padded pipe or swung to one side so the horse can stand with a wider stance and keep his balance better. Horses liking the divider swung over usually has more to do with standing wider than loading. This alone makes most horses happier in a straight load.
Also, when hauling, you can slow down, or speed up or carefully go around a curve, but you can combine braking and taking a curve at the same time. Slow down and /or use your brakes BEFORE the curve and then keep a steady speed going around it. You must slow down to a much slower speed if you approach an S curve. The horse will be balanced for the first part of the curve and will not be able to switch to the other direction very well so you have to go slower. If you haul horses too roughly very much, you will end up with a 'climber', 'leaner' or 'scrambler' that tries to climb or lean on one of the walls of the trailer. This is a real hard vice to ever cure. I have seen several horses back in the 60s (when only straight loads were around) that had open sores exposing their hip bones from leaning so severely.
Secondly -- If a horse leads properly -- not just follows when it wants to -- IT WILL LOAD! I have trained horses to load for 45 or 50 years now (and they continue to load if they are not hauled too roughly) by 'fixing' the leading problem.
I use a 'draw halter' or a lip string and apply 'light ' pressure until the horse takes a step forward and then instantly release the pressure after each step. I have never had it fail and that is with flippers, horses that have 'struck out at' and hurt people and horses that have tried to 'self-destruct'.
When I get time (I have to get a set of trail horses ready to ride this morning (and also on Saturday and Sunday) I will put together a post on exactly how I do it. The key is to apply 'light' pressure and not let the horse get reactive and 'on the fight'. It works every time for me and for others that learn to keep their temper under control and never yank or jerk the lead but simply use 'pressure and release' in a very calm and light manner.