What I do to try to correct this problem (please let me know if I'm approaching it the wrong way) is I'll make him stand for 5 seconds (at first) and if he moves...we start again at 1. When he stands for 5 seconds I'll let him take 5 steps forward...and then I'd ask again. I'll repeat until he stands for the 5 seconds without moving and continues to do so...and then I'll stop him for 10 seconds...then 15 seconds and so on.
It seems to work alright, but the minute I say enough and work on collection in a canter (he's really bad at that) and then I ask him to stop agian he gives me the same problem.
If I just trot him around he's alot better (but still will move around some) it's the canter that is the problem. It's like the canter charges up a battery inside of him making him want to keep going and going and going.
The saddle fits fine...his feet were clean, and everything was fine. He didn't appear to be in pain or anything like that.
I do, occasionally, ask him to get a head set when we stop, because I'm (hopefully) going to be showing English with him, but I make sure his reins aren't tight enough where that's the thing that is making him move. I'll go airplane style (feet sticking way out) to make sure I'm not giving him any commands that I am unaware of.
Any ideas or suggetions?