Unhealthy eating habits can contribute to tooth decay, but can also appear when the child's teeth and gums are exposed to any liquid or food other than water for long periods or frequently throughout the day. Natural or added sugars in the liquid or food are changed to acid by bacteria in the mouth, this acid then dissolves the outer part of the teeth, causing them to decay. I'd definitely get this checked out by a dentist!
Could be dental caries, caused by formula milk. Better get that checked the dentist quick so they can recommend proper treatment. Try not to let your baby fall asleep with the bottle in his/her mouth. That's what happened to my firstborn. Now she has two silver caps because she developed really cavities.