I have seen people but a rubber thingy hanging down from the halter at just the right height to let the horse chew on it. Keeps him busy.
And, you can try the "if you don't want to stand still then you can work" approach. Bring in pony, stand and ask pony to stand. If and when pony starts to move, pick up lead line (make sure you use a LONG one) and make pony move around you, like lunging him. Make him move briskly. Then offer him the chance to stand. When he can't stand anymore, ask him to work again. then offer to stand. Do this over and over, and it may take 10 or more times, he will eventually choose to stand. Then maybe no crossties will be necessary. Don't make it a "punishment" for moveing. Just think, "ok, you need to move, that's ok, but let' it be really moving!" Let Pony choose to either work or stand, nothing in between. He will eventrually chooose standing.
This is far more important than grooming, so give yourself and your friend time to do this and forget about grooming. When you get him to stand quietly for about 10 minutes, reward him with a light pet or some grooming or a treat and let him go, even if he is still dirty. Being dirty is no crime.