Many states have a seniors sevices agency that handles elder abuse cases and they would probably have helpful information.
This might even rise to a criminal act rather than just a civil dispute.
Reporting them for elder abuse might be the fastest way to shake the check loose.
I’ve checked with various levels of law enforcement and prosecutors in my area and they all say it’s a civil issue.
One lawyer I talked to said the civil route would be the best because he would be able to recover substantial damages that way that he wouldn’t collect in a criminal case.