I’m assuming you’re partly worried about the “Of COURSE I’m not flirting! Jeez, women today! Can’t a man be NICE blablabla”-denial if you bring it up directly?

That’s what he’s counting on. Use it against him.

Start referring to him as your “work uncle”. Do it in front of other people if you want. Personally, I’ve had excellent results with “bro”, though he might be slightly too old for that.

Completely shut down ANY talk about the state of your marriage (google Grey Rock). It’s creepy as fuck that he’s this invested in your marriage. You want to give a meaningless answer, then deflect. “Oh, we’re peachy! Anyway, have you seen the numbers for Q3?”

He wants to pretend that he’s not flirting, so you can play along. Make the very idea that you would be flirting with him ridiculous.

It’s not fair if you have to do this. It’s not fair if you’re the one who has to do the balancing act of keeping this professional while not offending him.