I, too agree with letting to drag, but at first I'd teach to lower head by downwards pressure in the halter. Also, it is a good thing to teach to untangle himself - when a horse is comfortable with ropes touching his hind end and good with following the lead, you can wrap a rope/lunge from one of his sides, around his hind legs, and up to the other side and then tug the rope and let him figure out how to untangle.
I was recently very grateful that I've taught my horse how to react on ropes that are dragging behind him or when he's tangled himself, when he managed to get loose during a training session in a larger field and with a really long rope attached to his halter. He cantered and bucked around the field, but did not panic, did not step on the rope and untangled himself at all the moments the rope got stuck over his hind legs - it is sometimes a really useful skill.
I have come a long way, to surrender my shadow to the shadow of my horse.