I would get a trainer for this, but have you tried a Halti? Or the type that goes around the dogs nose to stop him from pulling – Head Halti I think it’s called.
I would recommend a prong collar, but I only do so with EXPERIENCED owners, someone who really knows how to use one, and properly fit one.
Besides that, use high value treats only on walks, and continue with the stopping each time the dog pulls. And the Head Halti!