It's great to see your progress with Rex. I am sure I have read before that dogs tend to be more aggressive if they are leashed than off-leash, because a leash is somewhat confining, the dog feels he has to stand his ground more while leashed. I think just the fact you walk him so much and have him meet other dogs is working wonders for him. I also think others here have given you great advice too.

I sure look forward to seeing the pictures of your beautiful boy!