What do I do against a really low stance?

If i see a guy with a really low stance, i just get even lower than him. Im short though, and this may not be an option for you.

Be EXTREMELY heavy on his head, constantly putting pressure down on it. It will tire out his back and legs. Look for snap downs and front head locks.

All I can say to you is: Work on your reflexes a lot and throw your legs back and put your chest to his back when he shoots for your legs. That is all you can really do if this guy really has as good a form as you say he has. That way, you will have taken it to the mat and will have somewhat of an advantage due to your longer limbs. Shorter people have the advantage in neutral position and get a lot of their points from takedowns. Taller people can get their points from the mat with near-falls and, of course, end the match with a pin and a 6 point bonus.