I think that sometimes people think that their current GSD will be like their last. All shepherds are different. Although you've just now seen the aggression, I have a feeling that this has been building up since this boy hit maturity. Too much freedom for him. He didn't 'just' start resource guarding, he's been allowed to 'own' things for awhile. (This is just a feeling I have, based on what you've posted) So although neutering him is a great idea... you've got work to do. You have to undo what you've allowed to happen. A trainer is needed ASAP, too! Although it *could* be a medical problem and I agree to have him checked out right away, I have a feeling it's all behavioral. To re-home the dog, IMO, is the wrong answer. Although, if you can't manage him that would be best. However, if you do re-home him, get him neutered FIRST. All it takes is a lack of leadership with the "right" shepherd, and you end up with a mess! Trust me, I did it with my first one! Quite a bit of work to undo if it's gotten really out of hand and your dog is very hard headed. Do you have a crate or kennel for him? If not, you need to get one. He doesn't get an inch. Not one. You need some face to face help to learn how to deal with him and become the leader. Internet advice is one thing... but you're way past that now. If he sensed fear off either one of you... well, he's going to use that. A lot of it is your mental attitude, and I don't expect a 9 year old who just went through this to have it. So keeping him away from her for now is a must. He may get a wild hair and challenge her over anything. You've got a list of things now that you've got to get done, but if you start right away, I do believe you can turn this around. Most bad dog behaviors are human errors.