I would not take this advice. Whipping him with the end of the leash? Unless I'm missing something... It sounds like you have not been around an effective trainer who uses either positive reinforcement or prong (or a good mix).
I can't say anything about how you handle your dog because I haven't seen it. The trainer could be correct about handling your dog inappropriately. But I would not be smacking him with the leash. If anything, it sounds like he is just getting you to mentally shut down your dog. Your dog isn't learning anything, he isn't figuring out what the acceptable behavior is or being redirected, he is learning that he needs to shut down or he faces severe physical punishment.
Not only does this not teach him anything, I don't imagine it does much to build you and your dog's working relationship. It sounds like trainers who didn't work out before probably didn't know what they were doing with an aggression issue. I run into that quite often. Plenty of trainers that can fix basic bad manners, but don't know what to do about actual aggression.
Find a well-reviewed trainer who uses an actual training method, talk to them and get a feel for how they train, possibly observe a class of theirs, try a training session. But I would not stay around this "specialist" trainer. I understand that many dogs need different training methods, but there is a difference between a trainer who just doesn't work out, and a trainer who is not skilled enough to break down their training into successful steps.