BILL– Yes, the Goldilocks paradigm could be in play here—“this one is too hard; this one is too soft; I need to find the half halt that is just right!” I could be suspicious, though, that you need to approach the problem a different way. The issues (as usual) are connection and thoroughness.
It’s just possible that you’re trying to finesse things too much too soon. If you make your horse more solidly in front of your leg and working into the hand elastically but with a heavier elasticity, you may have more success. You don’t want to allow him to lean on you, but the contact/connection you have at the moment sounds too ethereal. Without over exaggerating, apply some muscle with your driving aids, and if he wants to run through you, (remembering lateral suppling aids at the same time) make strong enough interventions until he agrees to channel the extra energy into the bit.
Try convincing him to make a more physical commitment that permits you to make more overt manipulations without him breaking apart. As you can trust him to be more connected mentally, you can diminish the size/weight of the aids you’re using and end up where you want to be.