Early childhood tooth decay occurs when children frequently drink from bottles or sippy cups that contain sweetened milk, juice, soda or other sugary liquids, or often snack on sugary foods. If the liquid stays in the mouth for a long period of time, the teeth get big cavities that sometimes can’t be fixed. The sugar in the liquid combines with bacteria on the teeth to produce acids that attack the child’s teeth, which causes them to decay. It’s important to know that almost all liquids, including milk, formula, juices and other sweet drinks, have sugar in them.

As long as it isn't causing him any pain I would not do anything since he'll lose the tooth anyway. If he is experiencing pain you should get him to the dentist as soon as possible. They may be able to fix the tooth but chances are that they will pull it out. To prevent future problems you should make sure you clean his teeth at least once a day, especially at night.