My GSD has nervous aggression. Trainer helped with getting past people but dogs he is awful. What can i do?

Treats don't work. He's a big boy and already in trouble for biting someone while being walked by someone else. I hear all the time other people saying their dog has bitten but not the consequences. Of course very worried. He has got go much worse since my dog passed away suddenly in march. She was the boss and even though she was half his size he literally followed her around like a lost puppy. I have medical probs including crumbling spine so could do with him calming down as no one else walks him. The person walking him isn't allowed any more.