Why does my horse dance around while being tacked up?

I have owned my pony for a little over 2 years. He started to dance around while being tacked up last spring, then he stopped for a couple weeks and started again. I haven't even put the saddle pad on his back and he starts it. Its very annoying. He likes to try to bite me when I take off or put a blanket on and buckle up the front of it. He is a little mouthy. He's fine for bridling and grooming, but right when i get the saddle pad out or training pad and hover it over his back, he starts to swing his everywhere and walk forward and dance around. Its really irritating. I don't know how to fix this. He stands on the cross ties perfectly when I'm cleaning out his stall, he doesn't mind them. I've tried to back him up when he dances and slap him, but it doesn't work. He's good when I'm riding him though, he loves to jump and run. I just don't know why he's doing this and how to fix it? Any suggestions would be great! Thanks!