If you are just going out for the evening, I would just say, "I'm heading out...have a nice evening" and leave it at that.

However, if you are going out of town for a few days, you should probably tell him that you will out of town and that if something comes up, he can text/call you."

You do not have to get all wordy and provide lots of info as to what you are doing. You want him to wonder and be curious.

If he provide you w/info about his activities, thank him for the info and go on about your business.

Remember...he's a roommate at the moment and you've been fired as the spouse.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.