At what point do I tell my kid "Hey, it's okay, you can come out if you want to" without knowing for a fact if he is or isn't?

If you're not a shitty parent this won't be an issue.

I don't understand how people are having this much trouble with what is at its core an obscenely simple concept.

A kid raised in an accepting environment will not see being gay as something to hide, full stop, in the first place. If you raise your kid properly you won't encounter a situation where you have to say something like that. They'll know already, to begin with.

When you ask them as a kid if they have a crush on anybody, you say "Do you have a crush on any of the girls or boys in your class?"

When you talk about dating, you say "In the future, when you're dating a girl or a boy, you-"

Straight is not the default unless you make it the default. Don't make straight the default, don't do that "Aw is that your girlfriend" bullshit parents do with kindergarteners who are friends with girls, just normalize the concept that some guys like guys and some girls like girls and this isn't even remotely going to be an issue, whatsoever.