Should I worry about a cavity in my 6 year old son's tooth?

Yes, you should. While it is said that kids start losing milk teeth once they turn five, sometimes an infection can run deep down in the gums and might affect new teeth as well. This is why doctors insist on having your baby's teeth checked every six months. Good oral hygiene is necessary and one should start practicing it even before the first baby tooth appears.

Take the child to a dentist to see if the decay is deep and if your child needs to get it filled. Chances are, the dentist will recommend to leave the tooth be as it is due to fall out soon. Ensure that your child maintains a good habit or brushing and if he's ready, flossing. A diet of moderate amounts of candy, soda pop and chocolates will be good as well.

You could bring him to a dentist to get it filled. This will prevent it from further developing and minimize the risk of getting an infection as suggested by Avantika. I've heard of some dentists suggesting to simply leave it so I think it will be better to consult one for his/her professional input.