I'd start with trot polls, it'll help him learn to lift his feet then I would make a bridge or something and make him walk over it. Any obsticle that he has to get over both physically and mentally will help when its time to get into the trailer.
When you're loading him walk up to the trailer and just see if he'll go straight in. If he doesn't back him up, and try again, if he doesn't again make him work by lunging him then try again. It takes alot of patience and after he's even just lifted his front foot in back him away and reward him then bring him forward. Do it each time with each foot, once he'll leave a foot in stop him and pet him then back him out again and start all over. This way he learns how to back out nicely as well as getting in nicely. Just keep repeating the steps and ask for more each time and reward him with he's standing there. When he gets in give him a treat and let him stand there for a while then back out.