Raise your hands, do not kick. Kicking just gets their attention doesn’t work as well and doesn’t actually teach them! Raising my hands always does the trick
It’s an evasion to going forward and using himself correctly. Basically you’ll need a really strong leg to make him use his hind end correctly. once you get him moving off his butt, chances are his head will come up. He also needs to be moving off your leg and well, moving at a nice trot clip. If he’s not going forward, he won’t be using his butt and is more than likely just operating off the forehand.