If you do not feel totally happy (you seem to be a bit hesitant) you could always get a second opinion from another vet. Maybe try another practice?
Are you still in touch with the dogs breeder? Sometimes they can offer a second opinion (they know the breed better than anyone, and especially the family line).
If he does not even know who you are and does not respond to you then maybe his life is already over, like a person who has lost their mind they don't know anything about it. Is he showing any aggression? Sometimes dogs revert to very basic instinct, that is all they now know.
Whatever you decide make sure it is your decision, listen to peoples opinion but ultimately it is you who have to live with it, don't be bullied into something you feel is not right.
I know it is hard, I had a GSD in a K9 cart for 18 months, in the end I had to let her go - even though her mind was fine and apart from the paralysis she was healthy.
When all this is over then be consoled with the fact that you did the best you could possibly do at the time.
If you do have to make that final decision, if you wish to be at home then you can get the vet to give you a sedative to put in his favourite food (as long as he is still eating) then when the vet comes he will be already asleep and will know nothing about it.
Whatever the outcome I wish you the best of luck, we have all been there.
Thinking of you.