How do I get my yorkie to stop pulling on his leash?

Please, no choke chains. He is a small dog with a delicate neck. Such a device could really hurt him. Get a halti collar. It is a muzzle/collar combo that keeps control of his head at all times. When he pulls, the halti will cause his head to turn and thus, turn him. It won't harm him and he will quickly learn that pulling is not acceptable.

Keep him on a check chain instead of a lead. Whe he pulls says no and jerk his check chain a little. Always say no with a firm voice. Continue to do this and in a few weeks your dog will stop pulling. Another good idea is to use the word "Heel" and teach him how to walk on your left hand side. Try to socialise him with other dogs slowly.