this is all quite strange. You seem very cold about all this unless its just your typing style or you are in shock?
He has (if I've got this right) arranged to meet this woman months ago and has been talking to her via fbook and (possibly?) phone for that long at least. He was going to take her to a restaurant special to you two. He then made business meetings around this date so he had an excuse to go (or pretended he had made them, i can't tell), he didnt get his secretary to book the hotel room (perhaps because he was booking a double), he mentions nothing to you and obviously wasn't going to, she sends him a message about 'getting cold feet', you question him and he says that it was just a cpoincedence (so lies to your face) then changes it and says her parents etc were going, and you're NOT feeling insecure??
Why haven't you looked at the fb messages, if the parents really were going you'd soon see from his replies. If all the messages are deleted you'll know your H is (still) lying to you.
I'm not trying to be harsh but either you're in shock or well I'm not really sure but its obvious what he was up to to outsiders so why are you not even investigating slightly (eg read the fb messages).
Sorry but you also don't know its his 'first offence' as you say, it could well not be
Agree with everything WWIFN said, this is just the start I think.