Wow....this sounds like a very difficult situation and I wish I had some answer to help you with your predicament. I guess the best I can offer is...if it was my dog or one in my family, I would seek the help of an individual who really knows how to deal with this. Being bit intentionally is something I would not tolerate by any dog I might own. I can appreciate your anxiety regarding how unpredictable he can be and I might suggest your anxiety fuels the fire to a degree but it would be very tough to not have anxiety in this situation. Perhaps, in the mean time until a correction to his behavior is implemented, I would be extra thoughtful regarding your activity around the dog so as not to give the dog any reason to take this nasty posture...but I guess that is rather obvious.
I believe it is incumbent on your mum to take a position to cure this situation if as you state " she is his "person"". She should be all over this and monitor the dog's behavior around you and others...and at the first hint of any similar behavior your mum should be all over this dog....and in a manner which leaves no doubts about it.
Regarding the notion "she thinks that our GSD might think I'm at the bottom of the pecking order and this is why he picks on me " ...this might be all true or not...BUT..if your mum is the "leader" then it is her responsibility to take the dog to task to correct this unacceptable behavior...plain and simple. I believe I most certainly am my dog's "leader" and I can assure you, if my pooch ever displayed any aggressive behavior towards another human or dog ( which was not warranted ) I'd be all over my dog in short order and it would get the message.