Put the cat on the harness without the lead intermittently. Loosen it and let him familiar with some weight on his body. No rush. No punishment. Distract him with snack. He might respond by stand still, rolling, no move. At the end, your cat will stay calm and relax with the harness on his back. This step usually does not take so much time.
Use this style of harness instead of collar which your cat will easily slip and escape. Cat is escaping master!
