I SUCK at Clicker Training............

 If Jack was ever trained with traditional training, then he is what we call a "crossover" dog, and he may not get it.  Don't worry.  Instead of doing this exercise first, do something he knows well, such as "sit" and C/T when he responds to the sit cue.  That may get him thinking that the click means he did the right thing.  Next day, go back to trying this exercise.  Do NOT pull his leash.  Move quickly, but remember that it is the dog that makes the decision to move with you.  If he doesn't, oh well, you can just stand there looking bored until there is slack in the leash again (that HE put there) and then C/T. 

Or, you can try this clicker method, which is a bit simpler: http://www.clickertrainusa.com/llw.htm

If the dog is biting at the treats, get a "food tube" - mine come from Clean Run.  You can fill with baby food, etc. and the dog can lap his reward;-)

Yes, to be realistic, he may just be confused about what you are asking. Dogs who are walked in prongs/slips regularly and are *taught* LLW on them, are expecting the same situation to occur, and generally rely on you to give feedback about what is right, to avoid a possible punishment.

I would work on some easier exercises first, like some he knows, or teach a simple target, where he will touch his nose to your hand. Dogs love it, it seems to open them up, and it might actually make your LLW training easier.

How are you delivering treats? With a bigger dog, and I'm giving a lot of treats, I will give treats in an open palm and sort of just shove it into the mouth. It prevents nipping of small fingertips.

 Although I would definitely do some work on teaching him to take treats nicely - often shark-mouth is a sign of arousal or anxiety, which may be true given your frustration and his confustion.