First, if your son has a cavity, it should be filled, even if it's his baby tooth. Contraray to comon belief, baby teeth are important, they set the right frame if you will for the adult teeth, consider baby teeth a space filler. If baby teeth are not healthy there will be more problems for adult teeth.
Second, if you are not comfortable with your dentist, I highly suggest you to switch to another one. There is a dentist specializing in kids dentistry in Bells Corners:
Ottawa Pediatric Dentistry - Kids and Teens Dentistry - Dr. Mandana Nikoui and Dr. Christine Corbeil, although I have no personal experience with them, but my dentist recommended them.