if your lead foot is outside his you should be able to throw a cross of your own and beat him to the punch. if not block or slip the right while throwing a short right hook of your own.
you can slip the right and come back with a left hand over the top or to the body. or you can parry the right hand with your left hand and come right over it in the same motion, like mayweather was doing to ortiz.
the most basic thing you can do is catch it with your left and fire a jab back at the same time. like the right hook technique but easier and less dangerous, also less reward though. but if you are struggling to counter it thats where you should start.