Well, you've gotten lots of viewpoints and recommendations. As others mention, it depends upon the child and his individual reaction to dental work as to whether it's better to do it all at once or separate visits. Maybe they can do two at a time. My youngest son has had lots of dental work and surgeries and did fine with the sedation.
I do have a few related recommendations/suggestions. I would make sure they don't use the amalgam (aka silver) fillings as those contain mercury and can be toxic. Unfortunately, I did not know this at the time (and believed the ADA partyline that they were safe - they are not and that mercury leaches over time). My youngest son now has toxic levels of mercury that we are slowly detoxing. The problem is that some people just don't detoxify as easily/readily as others, thus some folks are more impacted. So, I would ask for composite (white)fillings - they cost more, but they are far safer.
As far as why so many cavities, for my kids the issue was that they did not have enamel on their baby teeth. The dentist tried to tell me that was because I had had an infection or took antibiotics while pregnant with them. Neither was true. I didn't learn until my boys were 8 and 11 that I had celiac disease (an intolerance to gluten (found in wheat, rye, barley and oats) that is genetic). Due to that disease, I had malaborption of nutrients, thus the fetuses didn't get the proper nutrients to build the enamel on the baby teeth (those teeth are formed while the baby is a fetus). So, I would ask the dentist if the enamel is the issue for your son and if so have yourself screened for celiac. Since it is genetic, if you have it (or the genes), then your children should also be screened.