Probably the most overlooked thing in teeth care is - vitamin C. It's effect on baby's teeth starts during pregnancy, and continues after the baby is born.
You can most likely stop the problem by giving him vitamin C in form of "calcium ascorbate" and NOT "ascorbic acid".
Also make sure he has enough calcium in his food, as well as enough vitamin D. Best to get these from fish, like sardines (make sure he eats the bones inside as well)