I would get him focused on you rather than the leash. Try walking with treats in your hand. Every time he goes for the leash, say "Look" and bring your hand with a treat up to your eye. If he looks at you, give him the treat and praise. Eventually, you can eliminate the treat but if he starts for the leash, you just redirect his attention back to you.
At puppy school, they also taught a technique to keep the dog focused on walking next to you and face forward. They had some kind of very long-handled stick/spoon with peanut butter smeared on the end. The idea is that you hold it in front of the dogs nose (kinda like the carrot and the horse). I never tried it, but it seemed to work from some people whose pups would not walk a straight line. It might work as a distraction from the leash as well.
One final thought...my forever pup, Timber liked to hold part of his leash in his mouth. He didn't chew it, just held it gently between his teeth. Maybe he thought by doing this he was walking himself? Perhaps if you trained Sca to do the same, he might just hold it rather than chew it????