what the others said. If he runs from pressure, or leans into the bit, he cant do that if you only pull one reign. Depending on how fast he`s running though, dont turn him too sharply, he could actually fall.
This may sound silly, but did you try to tell him to whoa. You have to say it like you mean it though.
Also, I know I`ve had a horse run away on me and my mistake was I had a death grip on him with my legs. I know it`s scary, but the more tense you are, the less likely you`ll get the horse under control. Instead of thinking about falling or trying to stay on just tell yourself `im in charge, im in control`
Ask your instructor to practise your emergency dismount and emergency stop (aka pulley reign). If you have those down solid, you`ll feel more confident should this situation arise again!