I agree, approach and retreat. Rub where he is comfortable, brush over the area he doesn't like then back to where he is comfortable again. You may even want to give him small piece of carrot every time you touch there to make a strong positive correlation. If your not comfortable with going back there you can use the end of a whip or crop or carrot stick to rub and retreat.
Worst case, if its safe for you to do so, put your hand just outside the sensitive area. Let him squirm and move around. When he stops you take the pressure off. It worked for me with a horse who was very abused and would freak over anything. I'd wipe fly spray on him with a rag and I just held it on him until he stopped moving. Same with the blanket. Sometimes they need to get over the "flight" responds before they think "oh, well its not that scary after all!", especially when their first instinct is to get away from it.
Also, keep in mind many horses done enjoy being touched there!
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Last edited by SlideStop; 06-12-2013 at 04:27 PM.