At 9 years old, depending on where his permanent teeth are, it is usually best to try and save the baby tooth since the baby teeth are what guide the permanent teeth into the correct position. Usually pedo crowns are recommended with a pulpotomy (when the nerve of the tooth is partially removed due to decay) or if the filling covers 4 or more surfaces. Honestly, I have seen cases that at 9 years old the permanent teeth are far enough along that no treatment of the baby teeth is necessary as long as the parent is aware that the tooth may abscess in the 6months it takes to come out. I have also seen boys that don't lose their baby molars until 12 or 13 in which case you would definitely want to take care of the tooth. If the tooth behind it is a permanent molar, then definitely address that immediately.