Do you mean he sits down and refuses to move or that he won't walk without pulling? (Sorry I don't know the harness.)
Yes, a little more info would be helpful. Some dogs really don't like a harness for whatever reason, but will get over it with some training (associating the harness with good things like food or play time), but some never get over it and basically "shut down" with a harness on - become very subdued, refuse to move, etc. Kind of like how some dogs react to a cone. If this is the case I would go back a few steps - get the pup happy to see the harness by giving him a treat when you bring it out of the closet, then when he voluntarily approaches it, then when he touches it with a nose or paw, then when you out it on but not do up, then put it on and do up but only have it on for a few seconds, etc etc. All of this over a period of time, of course, going slowly and only moving to next step when the pup is comfortable with the previous one. Eventually you get to the stage where you put it on and attach the lead and go outside. Just a short trip at first - a few steps, maybe, and make it happy and fun, with liberal use of treats. Eventually you get to where the pup is happy to have the harness on. It's not the Ned of the world not to be able to use a harness but they certainly are the preferred way to do on-lead walks as it avoids strain on the neck. Of course all of this assumes you are training loose-lead walking, harness or no harness. Anyhow maybe I'm not addressing the exact problem, if not give us some more info into what exactly is happening and we'll try to be more helpful.
