Honestly, I would ignore the dancing and moving (within reason, if the horse starts being truly aggressive, or something like that I'd find a pro trainer to help). The issues is simple -- he wants to move, you want him to stand still. When he moves, stay with him and keep on brushing like nothing is happening, gently but firmly, with rhythm (wear tough shoes so you don't have to worry about getting stepped on). When he stops dancing, pawing, whatever, count to three Mississippi seconds, stop brushing, praise him (maybe treat him or scratch his withers
), and then go back to brushing while he's standing still. If he starts fussing again, just keep brushing until he stands, count to three, praise...
Eventually (pretty quickly, actually, unless the behavior is terribly ingrained), the horse starts to understand that the quickest way to end the brushing is to stand still and allow it. Brushing will not stop because he chooses to dance around in the cross-ties. Timing is crucial to a quick result. As you work on his ground manners and he begins to see you as the "herd boss", I expect that your crosstie issues will get better as well.
Additionally, be sure that you have some kind of safety ties; whether quick release snaps, Blocker rings, or ol' reliable quick release knots securing your cross ties to the wall. ANY horse can spook, and horse tied up and spooking can be a major wreck darn fast. Safety ties are always a good idea. *Climbs down off soapbox*