If the EZ harness is slipping off, it doesn't sound like it fits correctly. The harness should fit snug but not tight.
I have two golden rules for training which I gleaned from a horse trainer that I respect:
#1) Is it safe for me and my dog
#2) Is what I am doing working? If not, try something else. Be creative, have fun, after all YOU are your dog's best trainer.
I would try some different tactics. Forget food treats for this one. His reward will be on a mental level. Wookie can quickly learn to pay attention and respect you while walking using these few simple little tricks.
Make sure your harness is properly fit AND safe - be absolutely certain that there is nothing in the throat area. This will require that YOU be quick and on your feet
and that you focus 100% of your attention on Wookie. It will require that you follow through EVERY SINGLE TIME until Wookie gets the idea. Don't jerk the leash, and above all, don't get angry. In fact, enjoy yourself, sing a song, whistle, etc. You want Wookie to get the idea that you are enjoying a nice pleasant walk, but it won't be pleasant for Wookie until he stops pulling on the leash.
Remember, this is a training exercise, not a regular walk - and every single walks should turn into a focused training exercise until Wookie is walking nicely. Start the walk as normal. BUT before Wookie gets to the end of the leash, turn and start walking the other way. When Wookie gets to the end of the leash he will have no option but to turn and go with you or he will stop being the puller and start being pulled.
I guarantee that Wookie will dash ahead as soon as he realizes that something went wrong, so be ready and before Wookie gets to the end of the leash again, turn and walk in a different direction. ANY time that Wookie is NOT pulling, quietly praise him. His biggest reward will be that he is doesn't have to keep up with your quick direction changes.
You will look silly and for the first few walks you probably won't make it out of your own driveway, BUT it won't take long for Wookie to get the idea. Continue with this until he stops tugging and when he is walking nicely reward him by letting him have a turn at deciding which direction you walk. But any time he starts pulling, just turn and walk in a different direction. You can also add a verbal command, to remind him not to pull - NOPULL would work. This is helpful once he gets pretty good at not pulling and if he gets excited and forgets.