Start with just a few minutes in the stall and work up from there. Lead him in the stall, tie him and groom him or clean out his feet or whatever. Untie him, walk out and close the door. Correct him if he spins or rears just as you would if he was doing this while you were leading him. When he settles, take him out, but not until he settles. None of mine stall well since they are out all of the time. I have to work up to being able to put them in a stall and leave. It is not natural for a prey animal to accept isolation AND containment. They have to be trained or conditioned to it. Small steps.