Help with a nervous horse...?

I sometimes have the same situation. I work at a farm that has a lot of horses like this. They aren't my horses, but I work around them and want them to trust me. Sense you clean his field and he feels more comfortable out there, start there. When you're out cleaning, talk to him and see if you can get his attention. I'm sure thats easy to do because if he's nervous, he's observing you closely. Eventually you could get to the point where you go and pet him for a little while when you come and clean the field. Once he's completely used to that and it's no big deal, he should be better when you go in his stall and you could start doing the same thing in there.