Walk on leash ** to teach or not to teach?

Hello everyone, I have a 2yr old male , american pitbull terrier, I owned since he was 7 months old, I was always told to let a pitbull pull (with a harness ) on his walks as he gets more excersice this way, lately i been pondering about teaching him to walk on leash or not, im definatly not a newbie , not claiming to be an expert neithter, most of my life i have owned strong big dogs , like mastiffs, gsd mixes and similar, my concern is if i teach my pitbull how to walk on lead , right next to me, will he loose his gameness. I appreciate everyones response, but i would rather hear from owners or pitbulls or who have expierence with this breed of dog. Thank you.