I know it's very hard not to jump the gun and just accuse him right this very moment, but keep your cool and do some investigating (casually and subtley..).

Have you met this other woman?
If so, you could speak to her in person and voice your concern almost by saying instead you are worried about him.. etc.. and does she know why he didn't bring you to his family meal. Not suggesting you are concerned about her, but almost pretend you're asking her advice and see what she says.

Good luck x