I think it's a very risky idea to force a reactive dog into submission. Especially if the dog is fear reactive. First of all this is not a dog that the OP has raised from a pup. This is a dog that they rescued four months ago. Working with a rescue dog, of unknown background, can be tricky work - and dangerous. It's around the three month time frame that I always saw problem behavior arise in my foster dogs, if they were going to have a problem behavior. It takes time to build trust. Usually a lot more time than four months. IMO and experience, forcing, poking, etc could easily lead to redirected aggression and loss of trust. Building trust is imperative. Yes the OP needs to let the dog know that they have control of the situation, but that can be done in better ways than forcing submission. IMO.
OP, I agree that a front hook harness is not the right tool for this dog at this time, and I think Hector's advice is spot-on. Most important is to build trust and show him you can handle whatever comes up, without scaring the bejeezus out of him. And to keep everyone safe while you do that. You know your dog better than we do. I've just been on the receiving end of redirected aggression and I could place no blame on the dog. His background was unknown and he was a newer foster. With time, and mostly positive methods, he became a wonderful boy and was placed in a family after long term fostering.