How old is your son? Are these baby teeth or adult teeth? I wouldn't pull any adult teeth unless absolutely necessary. Yes, the wisdom teeth could potentially fill in that space but there are complications with that. As the wisdom teeth come in, they don't move sideways into the space but rather, they tilt in and lean in sideways which causes problems with the jaw bone.
He is 9 and they are adult teeth. We are raw foodists and eat naturally. I am afraid the mercury could be harmful to him because he absorbs things into his body better than other kids do.
I understand your concern. There isn't any solid evidence to show that the mercury from dental fillings cause harm to children or adults when placed. If there was, it would definitely be banned. If you are concerned about it, I would recommend placing white composite fillings (which are basically plastic).