Is this horse new to you? How much of his history do you know?
I know of a retired race horse that gets real fidgety when they put him in cross ties-the assumption has been made that prior to a race he was cross tied and groomed and this is an old habit of dealing with the anxiety. Tied to a post, he's fine, or if you brush him while holding him. So perhaps your horse is aniticipating something that makes him nervous or excited?
Try just tied to a post or hand held and see if he still fidgets when you groom him. Or take him to another place to groom him and see if he still does it. It could be the spot he's tied in makes him anxious for some reason-perhaps a boss horse is in a stall right there and he feels vulnerable tied there, or activity in the barn isle behind him where he can't see well. Or cross ties are too confining for him.
Another thought on the same lines about anticipation-do you only cross tie before working him or saddling him? Perhaps he's not happy with the saddle being put on, or the "work", esp. if he's fresh out of his stall. Maybe a little liberty just before you tie him would help-like 10minutes to run in a paddock then groom and tack up.
Or perhaps the cross ties put pressure on his head through the halter on his jaws and he has teeth problems that are annoying him. If he has an abcessed tooth, that could be it (though I would think there would be other symptoms-like problems in the bridle or while eating, weight loss, etc). But it's an idea to consider.
To stop it, you have to figure out first what's triggering it. It could be dis-respect, but it could be a legitimate worry in his mind that a little more exercise, less grain or being tied in a new area could help. If nothing else works, then consider dis-respect, but my guess is he would start with other methods of defying you if that's his intention, not tossing his head.
Edited to add that disciplining a horse that's acting out of anxiety may make the problem worse, so I'd definintly give him the benefit of the doubt since his behaviour seems fairly harmless-he's not biting or striking out, kicking or running over you, pulling back. No, it's not ideal to have him toss his head-and watch yours so you don't bump noggins, but for now, work on finding the cause instead of disicpline.