Stay low and shade to your right. You'll stay away from his right hand-I'm assuming he is an orthodox boxer. By staying low, you'll hopefully get him jabbing down at you. This compromises his height and reach advantages and will give you an opportunity to slip outside his jab and shoot your right hand over the top. If you can get him throwing his right across his body, by being outside his left arm, you'll have him turning into your right hand.