Hmmmm.. he just sounds herd sour.
That's just gonna take time and patience to get him over.
Can you lead him in little circles in the barn to keep his mind busy then offer him his feed and leave him alone so he gets to munch out?
Like work, release, feed, work, release, clean feet, work, love, let him back out??
That's about all I have. I've never personally dealt with this issue ( I feed in the pasture) So, I'm just throwing out there what I would try first.
Be strong and courageous, and do the work. Don't be afraid or discouraged by the size of the task, for the Lord God, my God, is with you. He will not fail you or forsake you.
1 Chronicles 28:20