Hello,
Dental decay is the 2nd most common disease in the world in children. So don't worry about it. It's very common. You need to take your son to a dentist and get a filling done. At home you can teach your son to rinse his mouth after eating or drinking anything, especially sugary foods. Start using a fluoride rich toothpaste as well.
Next Steps
Go to a dentist and get the tooth filled.