See, I think he actually knows a lot of this stuff, he's just wilfully ignoring it. ... I just think he may be so rooted in his faith that nothing could supplant it
That's certainly the problem, and the reason why forums like this never lead anywhere.
I suppose I could try pointing out dendochronology, but I do expect to get a 'god works in mysterious ways' type answer - or 'how do you know?' once again.
Possibly, you can try another tack. For a long time, it was a very standard view within Christendom that God wrote two books -- the special revelation of the Bible, and the
general revelation of the world.
People of good will, of all faiths, who have studied the world have come to the conclusion that it is old.
In the 19th century, this was, as far as I know, totally unobjectionable.
Even at the time of the Scopes Trial in 1925, William Jennings Bryan
was not dogmatic that the days of Genesis were 24 hour days. He thought they could have been 24 hours, or they could have been millions of years, and more importantly, "I do not think it important whether we believe one or the other."
The existence of young earth creationism is itself a very young idea. And especially the idea that a young earth is somehow fundamental to Christianity.
What did earlier great thinkers of the Church feel about the evidence of the general revelation in nature? There is an amazing quote from Saint Augustine in his work on the
literal meaning of Genesis (De Genesi ad litteram) (#39):
"Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of this world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds to as being certain from reason and experience. Now, it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking non-sense on these topics; and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn. The shame is not so much that an ignorant individual is derided, but that people outside the household of the faith think our sacred writers held such opinions, and, to the great loss of those for whose salvation we toil, the writers of our Scripture are criticized and rejected as unlearned men. If they find a Christian mistaken in a field which they themselves know well and hear him maintaining his foolish opinions about our books, how are they going to believe those books in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think their pages are full of falsehoods on
facts which they themselves have learnt from experience and the light of reason? Reckless and incompetent expounders of holy Scripture bring untold trouble and sorrow on their wiser brethren when they are caught in one of their mischievous false opinions and are taken to task by those who are not bound by the authority of our sacred books. For then, to defend their utterly foolish and obviously untrue statements, they will try to call upon Holy Scripture for proof and even recite from memory many passages which they think support their position, although they understand neither what they say nor the things about which they make assertion."
Augustine recognizes that people (even non-Christians!) can gain true knowledge about the earth. And that they gain this through "experience and the light of reason" -- science, in other words. And "reckless and incompetent" Christians may foolishly contradict this knowledge with their improper reading of Scripture.
The modern YEC position, which has only existed for a few decades, is exactly the sort of thing that Augustine warned against.