He does not want to see the truth. He wants to believe the comforting lie we were all taught which justifies the war as a moral crusade. Nobody wants to see their heroes tarnished or revealed to be villains.
I had a bit of a problem with it myself when I first realized the direction the actual data was leading me, but I have never been able to lie to myself, so when I saw it was irrefutable, I just admitted to myself what really happened with the Civil War.
The "good guys" were not the good guys. They were actually the bad guys.
I long ago accepted the truth that most wars are about money. Taxes, access to a river or the sea, trading rights, rich farmland, oil, minerals, it comes down to money. Are we to believe it was somehow magically different 165 years ago? That people weren't just as greedy and venal back then? There is a reason none of this is taught in the government schools. I got all the way through college and didn't realize any of the financial aspects of ante bellum America. It simply was not taught. Once you understand it was the South that was generating the vast majority of the exports and thus carrying out the vast majority of the imports....that it was actually the South which was the MUCH richer region of the country when it was founded....suddenly it starts to make a whole lot more sense.
Reading the often scathing commentary of the English - who were real abolitionists - is especially enlightening. They saw right through the BS and propaganda about how the North was supposedly fighting to free the slaves (even though they openly said they weren't and offered slavery by express constitutional amendment which was also never taught in the government schools).
An 1862 editorial in an English journal commented, “They (the Northern white men) do not love the Negro as a fellow-man; they pity him as a victim of wrong. They will plead his cause; they will not tolerate his company.”