You really gotta ask?
He doesn't want you to know he is talking to her. Could be he is thinking of jumping, but more likely he is trying to avoid you Hulking out on his a*s. Like you did when you found out he was sneaking around behind your back.
Dump him and move on.
Umm, mainly because most girls don't want to hear about the girl being talked to behind her back. Just a guess. . . I suppose it can be tough, you're talking to all these guys behind your man's back and sharing it all with him, yet he holds out on you.