The only option your father has is dental implants that are biologically compatible with him. Definitely wouldn't want to install another metal that will poison him the same way.
Avoid dentures as they lead to the shrinkage of the jaw bone. My so called sorry sack of shit smoking father uses dentures and he has problems with them.https://en.wikipedia.org/wiki/Dental_implant
Yet the anti-mercury scaremongers were all claiming that even microscopic amounts of mercury were harmful.
One filling in a tooth of a child leaves the door open to corrupt dentists towards placing more mercury fillings without taking into account the LD50 of mercury. As it happened in my case and the OP's father.
Every little filling a pro-amalgam dentist installs does affect the dentist consistently by him breathing in the fumes released by the unhardened amalgam. Unfortunately there is still many unethical dentists still fascinated by mercury's amalgamation.https://www.ncbi.nlm.nih.gov/pubmed/20526950
No amount is safe is to drive the point home that every little bit has a catastrophic affect over the long term to someone. Mercury does not belong in our mouths.
Every time someone breaks a mercury light bulb and makes a big deal. I laugh because the mercury released by my fillings is much greater yet everyone who've I've asked to pay to remove them never believes me and takes the word of the nice and fake dentists. Against all odds I stand and alone I will need to eliminate this corruption.