Be careful not to reward him for stopping. I have seen many people lead a horse to a trailer; when the horse stops, they look back, pat the horse on the neck and say "nice horsie, don't be scared," or words to that effect. What that does is send the horse a mixed message at best, and simply trains him to stop behind trailers.
Try this. Lunge the horse for a couple of minutes to get him responsive to you. Lead the horse to the trailer without looking back, and without stopping. Do it quickly. When he does go in, reward him with a pat, back him out and do it again. If this doesn't work, there are plenty of people out there who can correct this behavior without beating the horse. Buttrope is the last resort, in my opinion. Too many opportunities to get tangled up, and cause a bad experience. I only know one person (certainly not me) that can safely and effectively use a buttrope, and he hauls horses daily, mostly in emergency situations, and trains horses to load properly.
When in doubt, get professional help.