When my mom was training her colt he got into the habit of backing when he didn't want to do something. It got so bad that he would almost be running backwards and it got kind of dangerous. So after talking to a local guy (who admittedly has somewhat old school views on horses) she put the long lines on him and worked him from the ground, and when he would start to back up to avoid the work she kept him backing and didn't let him stop until he basically sat down. Which was much safer than dealing with it from his back. After only a few times around with this he soon stopped backing to avoid work and hasn't done it since. This obviously won't solve the barn sour issue, but it might be helpful to get him out of this habit of backing which is pretty frustrating to try to deal with.

In general, the best way to cure a barn sour horse, is to make the barn a somewhat unpleasant place to be.  Saddle up and work the horse hard in a safe place near the barn.  Then when he is bug eyed and breathing hard, bring him down to a walk and point him down the trail/road away from the barn.   Pick a point not to far away and ride him to it and back, he must go to that point calmly and at a walk.  If he fusses, bring him back and work him some more until he can calmly go out and come back that distance.  Then when he can do that, take and tie him up and let him stand for a while.  You can take his bridle off and losen the girth.  After he's stood for a while, put the bridle back on, tighten the girth and take him out again.  When he will go your short distance without making a fuss and having to be worked at the barn, take him further away.  Wash, rinse and repeat until he will go without making a fuss. 

Taking some grain along and giving it to him at the point where you turn back may be helpful.  Also once the barn is in sight, never come back to it at a pace faster than a walk, emergencies excepted.  Also don't untack, put in stall/pasture or feed as soon as you come back.  Let him stand for a few minutes with the girth loosened, untack and let him stand some more before putting him in the stall/pasture.  If it is near feeding time, make him wait a bit for that too.  This way, he doesn't have as much reason to hurry back to the barn.  He's going to have to wait to get untacked, see his stall/pasture, get fed.

Good luck.

Spotted Pony

 There are excellent articles on equisearch about both barn sourness and teaching your horse to spook in place.  I used the barn sour article (a John Lyons exercise of putting your horse's mind and body to WORK when he gets sour) and it really worked perfectly.  It's not so much about making being at the barn unpleasant, it's rather that if they balk, then they will get put to work and if they behave, then they get to go along nicely.

 Annie, you don't need a round pen for the barn sour exercises.  All you need is a plan.  When he misbehaves, you just put him to work, turns, transitions, exercises.  If he won't go forward, you can do exercises backwards!