Oh you poor love. What a shock. I agree with all the great advice from PPs. Don’t confront him today. In fact, remove yourself from him to avoid an outburst.
Say you’re feeling really ill and stay in bed. Better still, leave the house and go to be with family or friends you can trust not to let on (maybe the family member who tipped you off?). Tell him they’re having an urgent personal problem you can’t discuss with him and you need to be with them (not an entire lie).
I know it’s hard with all the emotions, but focus on your objective now. It’s protecting yourself and your future. Even if (and can sometimes happen) you think there’s a slight chance you could overcome this as a couple, proceed on the basis you’ll be divorcing.
That means you’ll need lots of advice, information and evidence. And to be financially able to cope at the point one or both of you moves out, which could be immediately if he’s confronted. Honestly, that could take months, including getting a job and therapy so you can think straight. But, if at all possible, bide your time until you’re there.
If you confront him too soon, it could get worse. He’s clearly a blatant liar and not be be trusted. If there’s another woman who wants to be with him, she’s likely to encourage him to get every penny he can. You have the advantage as he doesn’t know you know. Hang on to that advantage.