Hi. My Boyfriend Has A Small Indent At The Back

Hi. My boyfriend has a small indent at the back of his head. He did not hit his head. It only hurts when he presses on it. He use to have brain cancer. He went for a scan about 4 days ago and everything was clear so I don't think it has anything to do with that. Just not sure what would cause something like that and if it is dangerous or not. He can't go for another scan, since he does not have money to do so at the moment.