Does he have an absolute FAVORITE treat?! Something really good. Chicken, cheese, etc. Put some on a stick and while you're walking, hold it out so the dog has to walk or move forward in order to get it.
Or you could teach the dog 'touch' so you could use a
clicker stick also... Victoria Stillwell has used this often. You could also try simply throwing the treats forward so he goes to get them.
Also have him have the leash on all the time... even in the house, just let him drag it around. Eventually he will HAVE to move. And he will realize it's not that bad.
First teach him to LOVE his harness before even trying to walk. Here is an example:
I would definitely get a hold on this now... you don't want a dog that refuses to walk. How unfun is that? I think any dog can be trained how to walk on a leash, some just take more time and patience and positive reinforcement than others. I started watching an 8 week old Rottweiler puppy and it took him 6 weeks of walking every day to get him to finally learn how to walk on a leash. We couldn't let him get away with it because we knew one day he'd be entirely too big to carry, LOL
I have seen puppy mill dogs who have never even seen the grass or any part of the world except a cage learn how to walk on a leash so I know you can get your little Yorkie to do it!!
Good luck!!!