Simple to cure. It will takes a few sessions, but try walk around a flock of sheep on the lead, he will want to chase them, but a good sharp jerk of the lead sideways while useing the command NO, this will soon teach him that this is not the done thing and the penny will drop, then when you have him walking around the sheep with him showing no interest then bring him ringht upto them, again he might want to chase, again a sharp jerk sidways on the lead will distract him and the command NO should be used. It will take time but you will him back on track.