imma go full crazyass theory here, i ve not read the manga ahead im deadass reading after starting the anime:
the reason his head its so hard and why him, his father and probably his ancestors had that scar in their foreheads, it's because they get a stone ore of the sun mineral they use to make the demon killer swords, but only the family members with red hair and eyes can withstand its power so thats why his brothers didnt have it, and that stone is the reason his head its so hard, so i think in the future he's gonna headbutt a demon so hard that he will insta-kill it