Our rescue GSD sometimes growls at our teenage son
We adopted a black GSD about 5 months ago - the vet estimates him to be 1 1/2 to 2 years old. He was found wandering around the dog pound and was very skinny - probably out there for a long time. We visited with him several times and observed him working with a K9 trainer that had been hired by the pound for evaluation because they thought that he had had K9 training and wanted to determine if it was agression training. My husband, i and our son all fell in love with him and decided that we really wanted him. He has been proven to be a great loving and fun dog. However he recently began growling at our son at night when he is tired and has "put himself to bed". He likes to sleep in our bedroom next to the bathroom door and when our son comes in the bathroom to get ready for bed he growls through the closed door. He has also growled at him when asleep in the living room at night when my son came down the stairs - bouncing as a young active teenager will do - but not even coming that near to the dog.
We loudly told him no and told him to go to sleep in his crate which is in a differnent room. He walked away and everything was fine.
We have read a lot about growling and how it can escalate to biting and are worried and not sure how to remedy this. The dog and our teenage son have been buddies and will play out in the yard for hours so we don't understand this sudden growling behaviour. Please respond with any insight or advice.