Sounds like he already lost his place as alpha dog. I don't think closing the door at night is going to help, they might get into a fight. If you want him to stay alpha dog you will have to remember which dog gets fed first, gets out the door first, which dog to pet first and invite him on the bed first (if the other dogs get on the bed, tell them to get off until Scooby is on the bed). You will also have to take a firmer hand with Shaggy to make sure he follows these rules, has he been to obedience classes? Do things like making him sit before going out the door, and after Scooby goes out, and sit before getting fed, so he knows you are the boss. Also, keep a closer eye on the dogs to watch how they interact and to prevent fights. It will take some patience and perhaps Scooby will not regain his place as alpha dog but its worth a try. Also, try not to make a bigger deal out of this than the dogs are, it won't be them who is upsetting the pack most of the time.