How is he doing it with his rear arm and catching you every time?
Is he doing it off one of your movements? maybe ducking your jab and following the move with it? or is he side stepping square and ripping up the inside of your arm? Are you too square to fast when moving in so he has that distance with his rear arm or is he the one going square? Nickoli's answer will be the one if it is you who is too square too fast. Questions worth asking yourself cause if you can answer them its easier to find the way to stop it.

If you get caught on your back foot though, you could be prepared and practice to pull your front foot back and out on an angle and immediately kick with your new front foot. That gets a lot of distance between your head and his original intention and line of fire.

A solid front kick off the 'new front foot' or a flying knee catches them as they turn to follow your new path; if you practice it and get it down first, cause you have gone out on the angle, distanced and they have to chase with their rear side more squared up so there is that opening up the centre of their arms route right there as they turn to follow.