FWIW I’d go with seeing if you can get comfortable around him first, as suggested here. I definitely agree that you have to be ready before going on this trip, and hanging out with the dude to get to know him would be a big part of that. Because the truth is, the two of you likely have a some stuff in common and you’d probably get along really well under other circumstances.
If you’re not mad at her anymore, maybe it might help to view him through the lens of someone who makes her happy; there’s also an opportunity here to make a good friend who you can trust to take good care of her and your son if something happens to you. If you do decide to meet up with him, you might even ask her about some things she thinks you might like about him so you have some conversation starters. And, it’s clear that there’s good will on the other side to try and mend fences for the sake of the kid, which speaks well to both his character and that of your ex-wife. They could have just said no and never mentioned it to you, or worse they could have been scheming to cut you out of your son’s life. A holiday invitation kinda says a lot about how they view you, when you think of it like that.
However you decide bud I hope you manage okay through all this, and I do hope you can find a way to be friendly with him. It’s incredibly strong of you to even be considering putting yourself out there like that. As evidenced by most of the comments here, that would not be a common reaction. Good luck!
I’ll throw my hat in for starting with DS9. I’m just finishing the final season with my partner now - her first watch - and it’s really cemented why its my favourite Trek. It holds up incredibly well.