So glad that your baby came back! the cat may have led him directly back home!
One thing you may want to start doing is to harness him with a leash whenever you need to open your door, and then continue to train him to sit and stay when the door opens.
I am waiting for a screen door to be placed on my front door in my apartment building. We are living upstairs , and have a balcony. Suzi comes outside with me supervised , and does not ever go down the stairs. Thankfully, she is scared to.
