My Son Fell At School And Hit The Back Of

My son fell at school and hit the back of his head on the concrete.... The teacher said she heard it before she saw it. He didn't black out or anything but left school early with a huge knot on his head. It went down and about a week later he hit the same spot wrestling around with his dad and the knot was huge instantly. I was worried but it went down after a hour with a ice pack. Since everything seemed fine til I noticed a bald patch in that area starting to happen. It seems like over the last few months it's a tiny bit bigger and if I press on the spot and rub it feels like wierd. Almost like a crack??? He freaks out if I mention taking him to the doctor for it but I'm concerned if he hits his head there again and something isn't right. Does any of this kind normal after a fall?