Hi there and thanks for the post
I have a few ideas to help.
Often the easiest way is to reprimand the second he puts the lead in his mouth. When he puts it in his mouth give him a firm tug on the line and also a loud GRRRR or AHHHH. If he lets go then praise him with attention.
If not go to him and remove it from his mouth. Don't make a game of it and if he struggles tell him off again.
You may also want to try putting some bitter tasting stuff on the lead like cayenne pepper, tabasco or sour apple. This will help him stop the chewing.
If all else fails you may have to go back to a short lead and slowly over a few weeks give him more and more lead. He can only have more lead when he has stopped chewing on the lead and is behaving
Good luck and please let me know how things are going.
Todd