Judi's string idea is a good one.
You can also try that when you have the leash in your right hand, make sure there is a nice loop so it's loose. Now hook your right thumb through your belt loop of your pants.
Chances are that when you have him on leash, instead of demanding eye contact, talking, shifting your body language - you're depending too much on the leash. Not paying attention, you check the leash. You've developed
your leash into your crutch.
Therefore, when he's not paying attention to you on leash, he knows you don't mean it unless he's getting a correction with the leash.
I know it makes sense to us that he's getting the correction because he's not paying attention. But to him - he thinks it's because you weren't serious the other times and THIS time you mean it.
With your thumb through your belt loop, you won't be able to use your hands to get your dog to heel. Therefore, you'll depend on everything else (like you do in off leash heeling) instead.
