Try just using a dull curry and soft brush your horse may have sensitive skin in that spot

I know someone that had a horse that was sensitive to grooming and she got a back on track saddle pad thing that goes under the actual saddle pad and in about two weeks, her horse wasn't sensitive at all. Maybe you could get one.

Your horse may have had a bad experience in grooming near his haunches. You can try fixing it by slowly reintroducing brushing towards the haunches until he fully accepts it. Hope that works!