What is the absolute worst thing that a cavity can do to you?

A cavity is actually a bacterial infection which can kill the nerve of the tooth. Once the nerve is dead it can form an abscess. If the abscess is in the top jaw, it can certainly create an abscess in the brain. If it is in the lower jaw, the infection can spread to the space around the heart. Get him to a dentist right away. An extraction is better than leaving an infection untreated and it is not expensive.