I have read lot about potty training by using "tethering" inside the house where the pup is tethered to you at all times to prevent accidents. I would think that would help with getting him used to the leash, too. I didn't use this method and not only do I have a pup that will sneak off and do his business where he shouldn't at times, whenever he sees his harness and leash, the chase is on! He loves going out to walk and to take trips in the car, but he always runs from getting his harness on despite his excitement. He will run downstairs and stand by the door, but still hop out of the harness as I try to put it on him. I will think he is securely fastened and then realize he has one leg out of the neck hole...
I have found the step-in kind is the easiest, once yours is big enough to wear one.