condition or he may have weak teeth. It's possible that you may not be brushing his teeth as well as you think. Milk bottles at night could also be a culprit. See your local dentist so he can be assessed correctly. Normally the decay in primary teeth doesn't effect the permanent teeth but this still needs to be addressed.
Rotting of teeth could be due to various reasons. Like snacking habits, diet composition, frequency of snacking, brushing techniques, frequeny of brushing. To see exactly what could be the reason and how could you fix it. I would highly recommend to see a dentist because Baby teeth are very important to maintain the space for permanent teeth.
I hope this helps.
Dr. Jatinder Sharma DDS