Over the years, my friend has gently acknowledged that some of my gay relatives are not eligible for an entry ticket to heaven. He's not the least bit mean-spirited about it.

He's not mean spirited so it came up.. how? If you're not at all religious and he's that kind of Christian than he doesn't think you're going to heaven either, so why did he mention your relatives? Also how is it not mean spirited to say, "Hey, Bobby, I believe in an eternal lake of fire, and Satan's going to be flaying your Cousin Jordy there until the universe winds down"? Mean-spirited to you, I mean. Jordy's not there to be Saved™, so what benefit is there to telling you that he thinks your beloved Great Aunt Sally's being tortured for eternity?

the gay jokes come out — causing the family to titter.

Ah, so his family sucks, too.

He uses “gay” as a common jab at many of our friends.

What is he, twelve? At least have the courage of your convictions and use the F-word.

This in itself would not concern me — our circle of friends often makes irreverent jokes, although not in front of children.

Ah, so your friends suck, too. Except for whoever is the inevitable closeted person in that circle of friends, who needs an intervention.

very, very likely facing the discovery of things about himself that his father might not be comfortable with

If the kid actually is queer, he knows. And he knows his dad is a homophobe.