Allow you dog to lead.... Let him decide where to go, and you let him lead you for a little while. He needs to get use to the harness, and feel comfortable wearing it.
I also use a retractable leash .Suzi is just 4 months old, and has not had any problems wearing a harness. She loves running around . Does you dog like to be on the grass? MAybe try a different type of surface under his feet instead of the sidewalk.
It takes time, and I do agree with your decision. A harness is better for a Yorkie to wear, and safer.