He's reaching for grass under the fence. Feed him his meals in a stall or drylot, THEN turn him out to graze. Might work, might not. We had ALL manes rubbing in the same spot, until I figured out it was from the 2 x 4 in the manger, made originally for cattle. I remove the board and the manes grew back.
No I haven't checked for lice or parasites I guess should. I once heard that when its time to deworm them they sometimes start to rub their manes or something...it's time to deworm him maybe that's it?? Umm we don't have grass it's winter so why would he be reaching outside?
Yes I agree that it might be time to worm. But also I would get some fungal wash and scrub the base of her mane where the skin is as she may have some kind of fungal thing going on. We have a old mare that we have to wash and scrub her good on her mane as she gets super itchy and that cures it. We usually do it once in the spring, and once in the fall.