Make sure you know what they are replacing the Amalgams with. Has he had biocompatibility testing?
Start taking vitamin C now so it can build up in the body before the day of procedure, and then keep taking it for at least a few days following the procedure.
Buy charcoal in the capsules, break them open and make a slurry with water (discard the empty capsules) that your son will drink the morning of the procedure. Take some more right before and right after the procedure. Then again that night. This will help absorb any SWALLOWED mercury vapor and particles. A slurry will work much better than swallowing capsules.
The most important thing is avoiding INHALED mercury vapor - make sure they give him oxygen or filtered air supply. The worst exposure from mercury is inhaling mercury vapor, especially from drilling it out. Leaving the office asap after the procedure is not a bad idea.