I would just tell him that you would appreciate more open communication with him in the future about things like this. I don't think guys naturally assume they have to talk to you about everything......they tend to bottle things up more than women, and then occasionally spill it when they've had a few. It's not entirely harmful behaviour, but it can lead to arguments now and then. Just ask him to be honest with you, and assure him you'll not get mad. I think if he knows he can tell you stuff like this that you'll keep a cool head, he won't hesitate to do so.
As for the ex-gf, don't worry about her. Honestly, she must be a pretty odd woman to want to try and out-of-the-blue revive things with an ex from six years ago. Yikes.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi