I agree that liars are the scum of the earth.
I wasn't happy about my ex being in contact with his ex but I didn't want to sound like a bitch so I put up with it. It wasn't until after I'd left him that he'd admitted he'd been shagging her while he was with me, so yeah, I'm not too keen on boyfriends keeping in touch with the ex's.
However, your situation is a bit different. Seems that you've already received some really good advice.
I say, stay calm ... perhaps think about how you really feel about him ... do you honestly think you would be happier without him?
A lie is a huge thing in a relationship and you need to let him know how hurt you are. But if leaving him by way of punishment for lying is going to punish you just as much then it's a lose-lose situation for you both.
I wish I could help further ... let us know how you get on
