What is the way to deal with a hard-hearted father, despite all the kindness shown to him, and can his condition change at this age?

Allah has commanded kindness to parents and made it one of the greatest obligations after belief in Him. Undoubtedly, honoring one's father and spending on him are forms of kindness and acts beloved by Allah.

A father's harshness or his negligence in spending does not negate his right to be honored. Allah has commanded companionship with a polytheist parent with kindness.

It is obligatory to advise the father gently and to seek help from those whose advice he accepts, and to frequently supplicate for his guidance.

Honoring one's father and being kind to him, if done purely for Allah's sake, is one of the quickest paths to Paradise.