If you feel a horse's upper lip, it feels like a man's '5oclock shadow'. This is because the hair is always trying to grow, but rubbed out as a horse grazes.
If you watch your horse closely as he grazes, you will see he uses his upper lip a fair bit, almost like a finger.
I knew a horse once years ago who did not use his upper lip. At all. He would curl it back and just use his teeth. Massive draft horse, very dainty eater.
He had a full blown mustache, 3 inches long. Curled down below his lower lip.
Hilarious. Wish I had a picture!
So no, probably nothing to do with his gender, but how he eats in the field. Just a quirk. :)