I've been training a horse that's almost the same way. She likes to move her butt away from the mounting block. At first I tried to walk her around the mounting block three times each time she did it. I got sick of it though since I shouldn't be the one having to do the work and it just irritated her.
I would make her do short (30-60 second lunges) every time she tried it. If she even moved her legs a little in the opposite way I would send her out and make her canter/trot in a tight circle, then bring her in. Tried mounting her again, if she moved her feet I would send her out again. I repeated this until she stood quietly for me to mount. It didn't take her long to figure out that standing was much easier.
I would do the same with this horse. When he moves, at all, send him out on the lunge line right after he does it. You may even want to keep a halter under the bridle and keep a lunge line or lead rope on hand. Make him work for a minute or so (hard) and bring him in. Do it until he stands nicely while you get on. It may take awhile but it's worth the effort and he will finally figure it out. When you do manage to get on, make him stand for a long time. I'd say 5 minutes, more or less. You actually may want to take a couple days and just do this. Mount, dismount, lunge, etc. for the whole lesson. Never let him get away with moving his feet, no matter how small the movement.
Best of luck, make sure to give us updates on his progress!
You can tell a gelding. You can ask a stallion. But you must discuss it with a mare. -Unknown