Good for you for working to find solutions to help your family and Mason! I am sure you are open to perspectives in dominance and in looking into behaving differently with Mason so he can live happily in your family with you, hubby, and baby.
You're probably actually however NOT dominant over your dog. If you were, he wouldn't feel he needs to be making decisions about which strangers to be aggressive to and when-- you would be making those choices as leader. (honest-- dogs may be people-aggressive due to weak nerves, all dogs DO bark at new folks, but, he IS making a choice that HE is the one who takes leadership position to screen strangers HIS way) Secondly, Mason, as the packleader, was putting your husband in his place, it sounds like. Mason could easily smell who he was cornering and growling at, even in the dark. Mason very likely felt he needed to remind hubby of his lower-ranking place.
It's hard to eval over a web forum, but it may be that like many GSDs, Mason needs you to ùse a comprehensive, all-inclusive, consistant daily program using key points such as dinnertimes, reunions with the pack, etc-- to show him in HIS language, where he ranks. No yelling, no hitting, no choke chains, no bullying... this is about using HIS language. It works, too! This involves eating before he eats. Going through doorways before he goes through them.(that is HUGELY important to dominant dogs-- they really notice this, even if we don't) Having him work for anything he wants-- food, waterbowl, toys, treats etc-- even affection-- have him sit, give eye contact, then WAIT.. until YOU release him. Look up NILIF on the web-- it means Nothing In Life Is Free. Mason may need such a program implemented by you. MANY of our dogs here in the forum did, too-- mine included!
This is not as hard/overwhelming as it sounds. Once you start, it becomes a habit. Mason will not only calm down (hey, there's much less stress when you are no longer at the top), he will no longer challenge hubby anymore. He may begin to look to YOU when strangers call, even if he barks a bit.
Or.. Mason may not need any of this, it is hard to eval over a forum. The cost for a good canine behaviorist (not trainer) would be worth it, if you have doubts or need guidance. They can be really very helpful in determining cause for behaviors!
The baby incident is actaully easy to correct-- the baby cannot be dominant over the dog, so the baby cannot be allowed near the dog when he eats, sleeps, or enjoys a rawhide. And it's a positive that Mason growled to tell the baby off, rather than biting. We all know canine lumps-o-pudding who would allow a child to reach for their rawhide, but many GSDs wouldn't.