Is he stalled at all or does he stay loose in the pasture all the time. If he is truly afraid of everything, it might be best if you didn't approach him with "monsters" at all for a while. There is another method you can try. If he's stalled, this might be a bit easier, you would just have to be extremely careful not to come to close, but take something a bit "scary" into the barn and mess around with it for a while, but completely ignore him. For example, take in a raincoat, stay a dozen feet (or even two dozen or more if that's what he needs to relax and watch, without panicking), from his stall, turn your back on him and ignore him completely and shake it out gently, or fold it, or hang it on a hook and "dust it off", play with it for 15 minutes, then take it and leave. Don't even make an effort to let him sniff it. If he doesn't feel threatened, he will become curious, wonder why you are messing with said thing. Do this for a couple days. When he no longer reacts to that, move closer, until eventually you are just outside his stall and repeat, again, don't react to him at all, or try to actively show it to him. If he makes a move to check it out on his own, hold it very still, or lay it down within sniffing distance and let HIM make the move. After a few times of this, with different objects, once he sees these odd things aren't out to hurt him, I'd be willing to bet he'll relax and be able to handle a little more pressure. You can do the same in a pasture, but it may take a little longer. The main key to this approach is not to push him, but to ignore him, and let him figure it out on his own. He will eventually have to stand up to more stress, if he is ever going to leave the farm, but this is a good jumping off point to build his trust in YOU, that you aren't going to bring anything to him that will hurt him.
"Keep a leg on each side and your mind in the middle"