Hmmmm I've done this with older horses....might work for him
Put him in a pen, round, square, whatever....
Put a longe line on him and let him move out however....hold onto him, ask him to move out if you have to,
Timing is imperative....
Position your body where you are (not in front of him) but ahead of his shoulder with him out on the circle and raise your hand at eye level to him and say 'stand'......if he stops and stays where he stopped great, DROP the line and walk over to him and reward him.....don't say whoa, say stand.
Continue doing this until he stands there EVERYTIME you ask....remember hand up at eye level (not in his face though) .....soon enough he will be standing for you.....if he moves off just do it again and again until he figures you out.....then you can progress to actually having him stand while you throw stuff at him (I bounced a bucket right off a 4yr olds rump from a ways away once and he just stood there.....the horse was 17hh, it didnt hurt....it bounced!) I don't recommend throwing buckets ( I was super testing a horse) but each time he moves, give him the command again and try again....it does work.....I had a horse out in pasture that was trained this way and one day I was snapping the lead rope on him and he spooked at the sound of the snap and ran....I put my hand up and said stand......and he did in mid flight
. Good luck
(haha as for the buckets.....well you never know when buckets are going to start blowing across the yard at your horse!)