While I understand re-homing a senior Grey is a sensitive issue, let's all take the opportunity to help solve this situation as best possible. Many pet owners would not reach out like this person has and would have taken the dog already to the humane society or worse put them down. From what the owner has written they are sensitive to their pup and do want the best for him.
There has been some good advice so far. I don't have much to add and wish you can solve this the best way possible. A muzzle is a very good short-term recommendation as he has already bitten someone once and you certainly don't want anyone else being bitten. Also the suggestion that perhaps he has a health issue may be very valid because if he is in any pain, he will be stoic but will react if he feels in way he'll get hurt.