thanks for all the info! It's really helpful when people actually take time to describe their problem in depth. I'll see if I can help you with this =)
You mentioned you don't have a trailer available to practice on. That's okay, first start with his ground manners. I have no idea how his ground manners are, so I'll start from scratch. (and believe me, this does connect with trailer loading) How is his leading? Does he walk ahead of you or lag behind? If he walks ahead, the quick way to fix this is to back him up if his nose ever passes your hip. If he lags behind, you'll have to teach him to give to pressure, which I strongly suggest even if he walks ahead.
If you aren't sure, have a trainer help you or even rent a few horse training DVDs or books to help you train your horse to give to pressure. He should take a few steps forward when you put slight pressure on the lead rope away from him, and he should back up when you put slight pressure on the lead rope towards him. So then if he feels pressure on the lead rope while you lead him (whether he lags behind or walks ahead) he should give to the pressure and come back to your shoulder.
Perfect his leading manners before you move on. Since you already introduced backing up when you taught him to give to pressure, perfect that also. See how fast/how long you can get him to back up with just slight pressure on the lead rope. Don't get him frustrated though, start with a few steps and progress from there.
Until you get access to a trailer, sharpen his leading/backing up skills. This isn't going to 'fix' his trailer loading problem, but it's a HUGE plus to help you fix it. Next time you have a trailer to practice on, see if he's nervous just walking around the trailer. If he is stop him about ten feet away from the trailer (you may have to stop him farther away or closer depending on how nervous he is). Let him look, and make sure you're calm as you can be. Do a few stretches, breathe deeply.
Pet him and talk soothingly. When you're ready, take a few steps towards the trailer. He might tense up, but that's fine. If he willingly takes a few steps, reward him by backing him up away from it and letting him stand. Calm him down, praise him. When he's completely calm, get him a little closer to the trailer, then back him away. Each time you back him away, let him stand and calm down, and praise him. Keep taking him closer and closer like this until you're at the trailer. Ask him to put one hoof on the trailer. Don't give in until he does. All you should be doing is keeping a little pressure on the lead rope.
When he offers you a step, reward him by backing away into his comfort zone. Next time ask him for two hoofs, and when he gives you that, back him into his comfort zone. Keep at this, inch by inch. Once he puts all four hoofs in the trailer, back him out into his comfort zone.
In the end, you should be able to load him without any pressure on the lead rope. He should back out now that he knows backing means he gets to be comfortable. If he doesn't back, use the same method. Ask him to back a step, if he does stop and reward him until you're out of the trailer.
This could be an all day project, depending on how well your horse wants to cooperate. don't give up and good luck!