I would be honest with him and tell him you saw the texts. I have a 15 yr old girl and a 14 yr old boy. I often go through their phones/FB without them knowing, and I would never tell them unless it was just this sort of situation.
I don't think it's TMI - in fact, I don't think it's enough. After all, forgetting your son's age for a minute, this is a 12 year old child we are talking about. You really don't want her and her parents turning up at your house complete with baby on the way!
Sit him down and explain STRONGLY that what he is suggesting to her is wrong - he know's that anyway, but I would hope the embarrassment of knowing his Mum knows may just shock him enough to hold him back from doing anything silly.
Good luck : )