Weird dent in back of head

So my son when he was like 3 months has a little bit of a flat head but it eventually rounded out but I was recently giving him a bath and I noticed he has a dent in the back of his head.. it’s not flat but as if he got hit with a golf ball and it dented his skull (didn’t happen needed a example of how it feels)

This picture isn’t mine but it shows how it is.. his also seems just a tad bit higher, I can’t take a picture of his because he has hair which makes it hard to see but you can definitely feel it