How can I prevent my Houdini-esque cat from getting out?

i would install a baby gate in front of your sliding door. your cat will still be able to jump over it to get outside, but it will give you and your room mate a few extra seconds to slam the door. you need to sit your room mate down and tell him that he NEEDS to be more careful when it comes to letting your cat out. tell him that his LIFE depends on it. hopefully it sinks in and he starts being more careful. as far as making your cat more happy in your home, do you have a cat tree? i would get one of those, as well as some cat grass to give him his own small garden right in his house.