What Causes A Cyst Near The Surgery Site On The Head?

Hi, my brother-in-law previously had surgery to remove what was supposedly a keratoacanthoma tumor on the back side of his head (he is bald) approximately one-two months ago. Currently, at the surgery site (or close to it), a marble-sized tumor (which hasn't broken the skin) has appeared. The plastic surgeon believes that it is just a cyst. Could this be something more.....squamous cell carcinoma (and actually cancerous)? My brother-in-law has only been speaking to a plastic surgeon, shouldn't a dermatologist and an oncologist be involved with what's going on with him. Thanks.