Cats and collars go together, but not cats, collars, and leashes. Unlike dogs, cats simply can’t accept the notion of being led on a leash, and tugging on a leash attached to a collar is (in the cat’s view) like hanging him in a noose.
If you have any hope of ever getting a cat to walk on a leash (and many cats never will), the only hope is the use of a harness, not a collar. A good flexible harness fits around the cat’s front legs and torso and, when snapped to a leash, is much less threatening than a leash fastened to a collar.