Originally Posted by Sharpie
Load him in. Then out. Then in again. Repeat for a while, then feed him in there and chill for a bit. Then load and unload a few more times for good measure. Repeat, repeat, repeat until it's no longer stressful or exciting or worth jumping about for him.
I would do this in combination with what the other person on here suggested...getting rubber mats.
Maybe try putting some hay near the entrance and just let him graze on it...without loading. Everyday move the hay a little further back until he needs to step up to get it. Eventually you should be able to feed him in the back of the trailer as usual.