OP, this is awful for you, but you must for your own well being reach out to people in RL.
You are very isolated, and that's not good at a time like this.
He is minimising and by you keeping it all from family and friends you are enabling him. He is counting on you to continue bottling it all up. I totally agree with other pp, he's telling you the minimum he can get away with. You need the truth, because your imagination will send you mad otherwise. He thinks if he continues to deny, deny, deny you'll accept his version of events and in his words you'll 'move on'.
In other words he's trying to isolate you and wear you down into submission. This can't happen.
A man truly contrite, truly sorry, would walk over hot coals to please you right now, he would be honest, answer repeated questions with total transparency, even though he knows it could cost him his marriage. A real man would have the guts to own his mistakes, and the respect for you would outweigh his own selfish needs.
His current behaviour to me, screams of guilt.
You say his family would go ballistic if they knew. Well, perhaps in this case, ballistic might be a good thing. It would blow the whole thing open, force him to grow up and be accountable, and to face, own and sort his marriage out in an honest way. Right now he's betting on you to protect him.
Stop doing that, insist on the truth, insist that he answers everything you ask. If he doesn't, tell people. He can't sweep his dirty little secret under the carpet. Shine a bloody big light on it.
You hold more cards than you realise.
Good luck.