I hate to say it, but Gizmo may not be the best choice to have for a service animal. They need to be calm and controllable and it doesn't sound like Gizmo is very controllable. Maybe test the waters with your neighbor about looking at an already trained dog. Because if Gizmo bites someone who reports it, he'll be put down, no ifs, ands, or buts. And no one wants to see a vet be forced to have that happen.

I hope that your dog is okay. I honestly don't think that Gizmo is going to make a good service animal. At least, not without a lot of professional training. He's going to need to have those aggression issues taking care of, and he's going to have to learn not to bite other animals, people, etc. Depending on his age and a few other factors, this may not be a simple matter to do. Do you know if he's been neutered? I know that with adults dogs, not being neutered can lead to some excess aggression toward other dogs, etc. Territorialism, etc, and if he's fully grown and has not been neutered yet, I'm not sure how much getting him neutered now would help with the aggression. My additional concern is if your neighbor was to look into getting an already trained service dog, there would be little to prevent Gizmo from attacking this new dog as well. If your neighbor really wants a service dog, the only option might be to find Gizmo a home we're perhaps he doesn't have access to other dogs, and then to looking to getting a fully trained service animal.

Unfortunately it sounds like Gizmo isn't right for the job. If he bites a human he will likely be put down, no questions asked.