Move to the horse's left side and pet his neck, or hug it. When you feel that you can halter him without him running away, then slip the halter on, pet him and lead him back to the barn. Resolve to work with him in a smaller place before you turn him loose in a three-acre field.
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You successfully caught your horse in the big pasture, but you obviously don't want to go through that every day. What are your options? Use the same lesson in the horse's stall, teaching him to turn to face you when you open the stall door. Then graduate to a small corral, then a slightly larger corral, then a situation with more than one horse and so forth. Work with your horse on leading lessons, and teach him the "head down" cue. The more comfortable he gets working with you, the easier he'll be to catch.
