Well said, this is perfect.It sounds like your heart tells you to go pick him up because you think he could make it in a more ideal situation. It seems this young man put Harley in a situation with roomates that Harley was not ready for. I think Harley could have a good life with the right handler but its going to take some more effort on your part to acheive this. tI have seen many shepherds that are one person dogs and they are handled that way and do fine. I have also seen many one person shepherds that over time became more accepting of strangers and much less of a liability. It takes time, training and love. I wish the best for you in the very difficult time.
If the dog is intelligent, loyal/loving to its person, and willing to learn. The dog is trainable and can be counter conditioned and desensitized.
It does sound like he probably was pushed too far. All dogs has thresholds. Some are a lot lower than others, you have to build up that threshold to a place where it is acceptable for you and safe for other people. The only thing that concerns me is that it sounds like he wen't full force with the bite and instead of a warning nip at the person (and hopefully only after lots of other calming signals and warnings were given!). That is something I personally would not take on as a project dog, but i am sure there are people out there willing to do so.
Call rescues, trainers who favor the breed, etc. and see if you can have someone give him another chance.
Otherwise my best advice to you if you cant, since it sounds like you love the guy.. Is to be with him for his euthanasia and spoil him rotten before. Dogs live in the moment, they don't count days, months or years. He will know that you were there, that he had fun and the energy from you. I'm sorry, this is so sad