W.C.
LOL. You poor thing, but what a fantastic little mind you have on your hands. He will go far!
They have magnetic alarms that you can put on his door. Once engaged, it will sound until the door is shut again. I would suggest putting one on his bedroom door.
NO joke here. Put his name and address on the inside of his PJ's I have been a 911 dispatcher for years and you would be suprised how often this happens. Far more found children are reported found than are reported missing and most can't be returned home until the parents wake up to discover just what you have discribed.
I took a call of a 2 yr old that didn't know where she lived, but the dog followed her out and they were able to get her back home that night because the dog had a tag with the address. Many times they don't find the homes until the next day when the parents wake up to find them missing, then they are already in "the system."
It sounds silly (horrible and neglectful too), but it happens to the best parents too (like in your situation). I like to plan for the worst and hope for the best. It sounds like any obsticales you put infront of him is only a matter of time before he figures it out. You have a smart cookie on your hands. Good luck. LOL.