What I would do-
I would make sure he can flex well to his sides, and stands still for that. Then I would go ahead and flex him, then "pretend" to mount him (while keeping his head flexed toward you) at whatever point he begins figeting/moving around, etc- I would jsut let him and fallow him around (as much as possible, depending on your position) until he quits moving. As soon as he stands still, quick completely and give him a rest for a few minutes. Then start over. (Depending on how bad he is, and how much time you have, may want to spread it out). The first few times you get completely on him without him moving an inch- I would just let him stand there and then get completely off and do something else.
If you keep his nose tucked toward you, he won't really be able to go anywhere.
The main challenge would be patience. 1) it's easy to hurry them and 2) they can (and will) make you think they can twirl around in circles forever. MOST horses get the idea quickly. On my horse, I make sure I can get on and off my horse from either side I choose (at a minimum, I also do random things- such as sliding off/on her butt and such that I can imagine stupid people trying- so that she is prepared to deal with them better than I do :) )