My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today.
It sounds like your dog has some separation anxiety. He likely is worried that his new family is going to leave so he is more watchful of you. Usually this happens when people actually leave but for him, he is just stressed being away from you.
It is natural for a dog to want to be with their master and from experience, it does not matter how many dogs you have, they all want to be in the room you are in. The scratching or destruction of doors does indicate that anxiety is likely an issue. One thing you can try is getting him a kong that you can fill with treats. Fill it with peanut butter or yogurt and freeze it. Take it outside and give it to him and go inside. It should help him get used to being outside without you since he will be occupied with the treat.
Another thing that might help is a DAP collar. These use a pheromone to calm a dog. They are proven to help with anxiety disorders. See one here:
Practice putting him outside and then opening the door immediately and rewarding him with a hot dog treat if he did not scratch, bite and carry one. This teaches him that you are putting him out but come back quickly. Once he seems to not do anything when you initially put him out, lengthen the time he must be calm outside for you to let him back in. Change the time as well. Make it 2 minutes one time and 10 mintues another, so he never knows if you are going to keep him out for an hour or for 2 minutes. It helps him stay calm for longer periods of time, just be sure you reward him when he is good. This is behavioral modification. Another thing that might help is to put in a doggy door. If he has the ability to come in and out on his own, he may be more inclined to stay out for longer periods of time.
To protect your doors, what I've found helpful is to get decorative sheet metal and place it on the bottoms of the doors. If you get the kind with a pattern, dogs do not like the feel of it on their paws and generally stop.
It will not be an overnight cure and will take work on your and your family’s part to be consistent in your interaction with him.
Another option is medication, which is discussed on this site:
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.