I just glanced through the posts, so I may have missed if someone else said this, but...be sure NOT to tie him before closing the but bar. Always close the but bar and then tie, so that he is not tempted to freak out, pull back, break the tie, hit his head on the trailer ceiling. They seem to be much more accepting of the but bar securing them (when they are ready) than being tied in an open trailer.
Everyone else's advice is really good. Just stand there with him once he loads, and remind him to whoa.. gradually increasing the time he spends in the trailer. And I definitely cannot say enough for the trailer being associated with a special place where they get to eat. My horses LOVE LOVE LOVE the trailer, and their training involves learning to wait until they are given permission to go in. (there is often, but not always, hay in our mangers. Our horses also like going new places a lot) :)