Are you a southpaw? Is he a southpaw? That makes a big difference. My advice is assuming that both fighters are left-leg leading (not southpaw). The shorter fighter must use angles and counter-punches to be effective.
Ex. If the guy throws a left jab, you move in and to your left countering with a left hook or jab to the chin or temple. Of course his right side ribs are unprotected so go for a kidney punch. This takes a lot of practice so start working with a friend.
Remember if you have good hip rotation and footwork, you can actually have an advantage. Be careful!