Mam according to your description it appears to be early Carious lesion. That is cavity. It needs to be examined by a Pedodontist and treated also as soon as possible to prevent it from spreading.
Dr Swati Karkare. Pediatric dentist. CHILD SMILE CARE.
Next Steps
take your son Ko Pedodontist. continue using toothpaste and brush to clean his teeth.
Health Tips
in case he has a habit of milk feeding during sleep then discontinue it as soon as possible.