Aggression

Hello. I need some help please. I have had dogs all my life and have trained them to be loving members of the family. However I have a problem and I have no idea how to fix it. I live with my 83 yr old father. I have an 8 yr old shepherd mix who is wonderful. My father adopted a 2 yr old Old English Sheepdog 8 yrs ago. "Moe" was returned to the breeder after his owner said he just got too big and strong for her to handle. Moe was left in the garage and was not socialized or trained. He didn't even know how to play. From our first day with him he bonded with my father. But for the last 8 yrs I cannot hug or even pull up a chair next to him without him going into his secret service mode. I swear the dog would take a bullet for him. Moe will get this very intent look and race over to get in between us. I keep eye contact with him and tell him to "back off!" It helps a little but not enough. He has bitten me three times; the last one about 3 weeks ago which my father does not know of because he said that he would put him down if he did bite again. And please, don't tell me to put him down. In my head I know I should but my heart strongly disagrees. But when I have him by myself and my dog he's wonderful. I can do anything with and to him and he obeys me. And I know Moe loves me. It's just when I get close to Dad. And it's only me he's aggressive with. Anyone else can hug him or get close to him and he could care less. I've never hit any of my dogs and I only use positive reinforcement. I've told him over and over the command to backofff needs to come from him but he's not consistent. Is there anything I can do to stop this aggression towards me? Thank you!