Guide with your legs and use your outside rein
Hey :) I find that using guide poles are help full, in conjunction with a couple of ground poles before it. I always warm up over a couple of low to high cross poles. I would also take alook at my self as I ride it, am I sitting correctly, not putting too much pressure into one stirrup than the other, am I not giving enough leg support in the side he's running out on. Perhaps in the case of the later, hold your riding stick on the side he runs out on. Hope this helps, best of luck!
As my trainer says to me, tube of tooth paste. Push with your legs and use your reins when needed. If that doesn't help, take rails and put them on either side of the jump, and make the narrow at the middle of the jump.(resting them on the top rail so the make a "v") so your horse can't run out