Get it taken out as soon as possible, by a reputable dentist who does amalgam removal, with a dental dam and oxygen mask. The stuff is horrible. Happened to my son too, and effected him for a long time.

Give him powdered charcoal everyday in water, have him swish it around his mouth for 30 seconds too, to absord the mercury and pull it from his blood stream.

You might even want to consider Chelation to get the rest out. But Clay and Charcoal are also good options. Chelation is a bit strong.

You can also pursue damages from the dentist, as amalgam fillings are highly toxic and you didn't approve them going in his mouth.

good luck.