I agree with Cinnys Whinny. Lead changes are not as easy as everyone says. You need a pretty solid horse, that bends well, works off your leg cues, and side passes well. Just a solid foundation. If you have a round pen, put him in there and ask for a lope going right. See what he does. If he is in the wrong lead, speed him up until he figures it out. Let him lope on that lead for a while. I have the same problem with one of my horses. We will get it figured out, but it is going to take time. On the trail, bend our horse around to the left, real tight. Make a couple turns, explode him out of it. He should come out of it in a right lead. It is tough. Good luck.