An exercise I've been doing for a couple of days with a tense horse is to put him in the pen at liberty. Let it walk about, sniff, roll etc for a few minutes. Then approach and greet with the back of your hand. Let him move the last inch to touch your hand. Then move to the horse's shoulder, facing same as the horse and a few feet off, whatever distance both of you are comforable with. You may even have to move to the middle. If your horse moves out to the rail, just stay aligned with his shoulder and mimic everything the horses does. If he moves his left first, you do the same, as best you can. Keep your arms at your side and glance at where the horse's head is. If he stops to look outside the pen, you do also, where he's looking. At first the process may leave him a bit confused and that's ok. Or, you may see his head beginning to drop into a more relaxed position and maybe even licking and chewing. 15 min is enough then without looking at him just turn and leave. The horse I did this with also yawned so hard 4 times I wondered if his jaw would break. Plus the licking and chewing. His eye was soft and relaxed. The second time it was in the pasture, just walking together. He let me much closer this time. If he stopped to graze, I bent down and wiped the grass with my fingers. The difficulty you might encounter is the shoulder alignment. If he picks up speed your behind the neutral line running thro the middle of his shoulder. If he wants to back or reverse direction you've gotten a little ahead. It become a bit of a dance getting it right. When the horse relaxes it's a glorious feeling.