If there's one thing I cannot tolerate, it's being lied to, ESPECIALLY by one of my close friends or family members. I can deal with the truth FAR more I can being lied to. Fortunately I've never been in that position.

I think, IMO, maybe he feels he couldn't tell you because you'd be hurt that he was spending time with whats-her-face, rather than you. I would level with him and tell him how you feel.