If he's kept in a stall, put a solid step stool in there with him. Not one of those little ladders or anything he can get his leg or hoof caught in. I did basically the same thing when I was desensitizing one of my previous horses to plastic bags (like WalMart bags). I hung them all over his stall. Not to mention wrapping one around his feed bucket, and hanging two more from the handle. He learned real quick he didn't need to be scared of them. I knew it had worked for sure when I walked in one morning and he had ripped most of them down and had stomped them into his shavings. He got to the point that one flying in the air on a trail didn't even phase him.
Also, keep walking him by it. Work on getting him a little closer everytime you pass by it.
I'm not so sure I'd put it in his stall, depending on how afraid of it he is. If he's extremely afraid of it, put it on the outside, right in front of his stall door, if he's able to view it from there. Also, yes, as charlicata said, walk him by it constantly. When he's comfortable being around it and sniffing it comfortably, start kicking it and stepping on it so that he knows what noise it makes when he touches it or steps on it. Be patient, and good luck to you!
Heh I'd put it in his stall too. Or in the pasture right by his hay pile or water trough. We had a horse that was terrified of pool noodles.... after a night of six of them in his stall he would pick them up and toss them around.