Start at the trot, go down the quarter line, and ask him to change bend and yield to the other direction, changing rein when you get to the other end, and repeating in the other direction. After doing this a few times, while asking him to yield, ask for the canter (still of the direction you'll turn at the other end). Then come in at the canter, trot, change bend, yield, and pick up the other lead. Eventually, come in at the canter, change bend, yield over, and give a bump behind the girth on your outside to ask for the change.
A good way to start is to get him at a canter, make your cross to switch directions. And break him into a trot to get his other lead. The smaller amount of trot steps you can get to get him to switch. Then set up a pole or crossrail in middle of ring and canter over it, switching his lead over the jump. And soon enough you can just take away the jump and he can switch it on his own. It may take months or even years. Depending on your horse and such