Could it possibly be painful for him to pick up his left lead? I would look into that, there may be something wrong. If he is such an Angel about picking up his right lead, but is so strongly rejecting picking up his left. Back problems, saddle fit, leg issues, feet ect.. Also, every horse has their good way and their bad way. For instance my 23 year old arab mare's good way is her left. I turn her around and ask for a right lead, oh boy.. haha Actually I've been working a lot on canter transitions over the last few weeks with Dandy (said horse above and horse in Avatar). She's doing GREAT! It's taught me a lot about working with horses and picking up the canter transition. She didn't even know what a transition was a couple weeks ago, it was trot, faster trot, trot that is going to bounce you out of your seat, canter! Now it's walk, cue, canter
My suggestion (as long as you have checked him out and there is indeed no medical issues). What I have to do with Dandy sometimes, to get her to pick up her 'right' lead, contradicts the actual cue to get them to canter. You know, the correct way to ask her to canter I'd have her collected, and head bent slightly to the inside of the arena and cue her with an outside leg. Sometimes, if she won't pick up the right lead correctly for me though, I will slightly over-bend her head to the outside of the arena and use an outside leg and seat. This sort pushes her into that right lead, I've made it easier for her to pick up that right lead. As soon as she picks it up, I reward her and bring her head back around to an inside bend. After a few strides, I stop her, then ask her correctly to pick up the right lead again (head bent inside, outside leg). If she fights me, I stop and again push her into that right lead, reward, halt, and then ask her again correctly. Usually after the second or third time she does it and every time I work with her it's becoming easier and easier. Good luck!
**And wow, just realized I was on page two.. -headsmack- >< Sorry about that, he's just being green. Not in pain most likely. My suggestion should still be valid though I believe! I know it's worked with Dandy. This gives me a look at what I'm about to get into though next week I start training my three year old filly how to canter. We've mastered the trot! Oh boy, though. I'm sure I'll be posting again soon! haha