Another trick in the roundpen is to put some pool noodles, lead ropes, and a few other toys in the round pen and leave him there for a couple days. Feed and water him, but don't let him out of there til he gets over it. Then, when he is used to things being still and in the same spot, move em all around and mix it up, then do the same thing. Last step was already mentioned, have him follow a rope dragging on the ground and progress to where he can have one drug anywhere around him without taking off.
My friends use my horse and I as guinea pigs and rope my horse's hind end and butt, but they don't pull the loop tight and let her walk out of it. Its pretty awesome because she is getting used to something touching her out of nowhere and not reacting to it!