Talk to Pediatrician on 5 Years Son Has Cavity.

My son is 5yrs old his teeths are getting black stains which is not going by brushing his teeth. He has cavity in his lower molar, whenever he eats sweet or cold things he complains about pain. We don't want to take him to dentist in this situation. Is there something we can do at home? I brush his teeth daily before going to bed at night. I'm so tensed seeing his teeths going bad day by day. Plz help. Thanks