You say that he is only going to be using his left as its more fair, so I take it he has been at it longer and is more experienced. More than likely he is trying to work on his left hand punches/combos, or one defense and left hand counters. See, sparring is not fighting, it is mutual training and learning. I would suggest you work on your jab (while simultaneously working on protecting your self while jabbing) You can work on getting past the jab to fight in close - and I'm sure your friend will help you work on not getting left-hooked while you do so.