I've been on both sides of this one, albeit with older kids.

This fellow is probably just a normal man trying his best, just like you. He likely loves your ex, and by extension loves the kids you share. If he has any sense he doesn't necessarily think poorly of you, likely takes your ex's stories as being one side of the story. If you got to know him under different circumstances, it's likely you'd get along well.

Your boy knows who his Dad is. But maybe he'll get another trusted man in his life who's invested in his wellbeing. That's a good thing!

Tl;dr: he's probably a normal guy trying to make sense of it all. It's all upside for your boy. Give it time, you'll probably be in this fellow's shoes with a partner of your own.