Autism and Escape

My friend has an autistic son that keeps getting out while he's sleeping and they live on a main road. Does anyone know how to teach him this is bad or something he can buy to help secure him in. The cops and child services are involved and I don't want him to lose his boys he's a new single dad. He tried a door alarm but it didn't wake him.

    He'll be 4 in july. A fence isn't possible he rents and there isn't a yard. He had a door alarm on the door and he stood on a chair and turned it off unlocked the door and got out.