It is actually more difficult for a horse to see a decipher the height and width a vertical fence, rather than that of an oxer or a series of fences.
When a horse approaches an object, it will actually go out of focus and (depending on the object) eventually disappear from his field of vision. He also has poor depth perception - and I can relate this to my sister, who has zero
depth perception, and does showjumping.
Imagine what the horse sees when trotting or cantering to a fence. He relies on you to get him there, and teach him to see those distances. Vertical, single fences have no depth. My sister tells me that these are the fences that scare he the most on course, because she can't see the distance very well. The horse may not be able to see the distance or the height
very well, and this causes some to over-jump, take off early, refuse, et cetera.
Now, your guy. First, I would check for any vision problems that may be hindering him furthur than a regular horse. Second, check yourself - are you letting him have his head to freely adjust his vision (does he have a poorly adjusted martingale or something of the sort on)? Are you hestitating, or to the opposite, being aggressive? Are you being his confidence, his leader, giving him a reason to trust you?
Set up the fence with things like trot poles (or canter poles), ground lines, anything to give it some depth, and then slowly take that depth away until he is more sure of himself.
And remember to be understanding and empathetic - when a horse refuses or does something silly over a jump, he has a good reason for doing such. Don't get angry, don't get passive - just reassess, retry, find the root of the problem, and approach it. It already sounds like you are doing such though! ~
Check out these articles. I also saw a video a while back that simulated the horses vision as it approached a jump, but I can't find it. I'll try to, though. Wikipedia - Equine Vision Suite 101 - Horse Vision