Most dogs that are overly aggressive are responding to fear. We see this most often at the veterinarian office, grooming and going to the kennel.
When your dog exhibits aggressive behavior and is not responding to normal correction, walk away from him.
Trying to grab his collar may illicit a nip or bite because he may misinterpret your intentions as trying to hit him. This is especially true with rescue dogs because most of the time, you have no idea where or what situation your dog has come out of!
After a brief time out, then you may try again with the task. A gentle leader can be a wonderful aide in training on a leash.