I would strongly advise you to NOT just turn your boy loose with your parent's 18 year old cat, that is a recipe for disaster. In general it takes weeks to properly introduce two cats to each other, so if you don't have the ability to separate them, you may need to rethink how you are going to manage your trip. You may be one of those rare, lucky families where the cats just naturally hit it off, but I would not count on that...