Baby teeth (or primary teeth) are just as important as permanent (or adult) teeth. They should be well cared for, and the best way to take good care of them is through good nutrition. Children who eat a diet of healthy, nutritious food generally do not have problems with dental decay. Sadly, many children's diet nowadays leaves a lot to be desired. Junk food (and this includes not only fast food from McD's and the like, but also any processed food high in white sugar and white flour) is very damaging to both primary and permanent teeth.
Pain will not be felt until the nerves are affected (either exposed or inflamed) but don't wait for that to happen. Go see a dentist, change to a more nutritious diet (plenty of fresh fruit and veggies) and research the findings and recommendations of Dr Weston Price. I would also highly recommend Vit D supplementation in the form of cod liver oil.
Also, if this is to be his first visit, check out my hub on Your Child's First Visit to the Dentist to make sure he has a non-traumatic first visit. Hope this helps.