My guy initially wouldn't stand for me at the mounting block either. What I found worked for him was whenever he moved without being asked, I moved him in small, tight circles (he's 19hh so not easy for him) for about 3-5 circles. I would then try the block again. If he moved, I moved him, in small tight circles, again for 3-5 tight circles.
We only had to do this for maybe 10 minutes, one day, 4 months ago. He now stands like a dream. He stands while I go and get the mounting block, my helmet, etc. He will not move until given the okay to walk on.