Maybe due to bottle feedings. Avoid sugar in milk . Avoid sweet and sticky food stuffs. Visit your dental surgeon soon

If a child is having a habit of bottle feeding and child is keeping bottle in his mouth for a longer period of time this will cause decay of teeth.

It may be due to multiple reasons like poor oral hygiene , incorrect feeding patterns and problems in the tooth structure etc Please consult a pediatric dentist at the earliest

He has early childhood caries I. E due to cariogenic food and poor oral hygiene and requires treatment & some lifestyle changes like.... 1. Please,  maintain his oral hygiene by following proper brushing technique as Roll- Roll method using fluoride containing toothpaste in a pea size amount only twice daily. 2. Gargle his mouth with water after every meals to maintain oral hygiene. 3. Concentrate on proper  healthy diet for child. 4.avoid artificial sugar containing foods. Also, Does he has pain with any of his teeth? If yes, please let me know. Otherwise can you please,  post his clinical photograph of the teeth in question to suggest exact diagnosis & treatment plan for him.

Does he feed by bottle feeding

This is a condition called early childhood caries which is mainly due to sleep time feeding practices. Treatment will depend upon the extent of the damage caused by the decay. Pediatric Dentist will be able to assess it clinically and tell you the treatment options.

Please consult pedodontist(the specialty of dentistry devoted to the treatment of children).