This is usually an easy fix. You need to lunge him when he starts moving.
When he starts to fidget when you're mounting, grooming, cinching, whatever, send him out on a short lunge line and make him work HARD for a few seconds to a minute. Bring him back and try whatever you were doing again. If he moves his feet, send him back out. Keep doing this until he learns to keep his feet still. It may take him 2 times to figure it out, it may take him 20 times. But he will get it eventually. Be consistent as well - he may understand it one day and forget it the next. I did this with my mare and now she never fidgets. They quickly learn that standing still is much
easier and will gladly mind their manners.
Some people lead their horse around in circles three times when they do it, but really you'd be the one working, not the horse, and it isn't nearly as effective.
Best of luck
Keep us updated on his progress.