Agreed with PaintLover, make sure first there isn't a physical issue going on: stiffness, weakness, pain of some sort, even a tack issue.
Is he green? Or is this a newer issue? Is he as balanced at the trot to the right as he is to the left? If not, building up his muscles at the trot and making him flexible will help out his canter quite a bit.
Originally Posted by UnrealJumper
our leg signals still need some work, but other then that he's a really good boy!
Fixing this will actually help out quite a bit. Remember that the first beat of the right lead starts with the outside (left) hind. So you want to orchestrate your aids so that will step off his left hind when you ask (which means you ask right before he picks up his leg). Before he can do that he needs to be able to move off your leg correctly. If he's not doing this well, IMO it's easiest to get a horse to do this starting with turns on the forehand (so he gets the idea of moving his inside hind over when you ask) and eventually a leg yield. In a leg yield your horse will move his inside hind over... which means the next step will be with the outside hind. Or, the first step of the canter. So when I have a horse that has difficulty picking up I lead I will often leg yield over, then immediately ask for the canter.
If a horse is in the habit of picking up the wrong lead, even setting him up for the correct one won't necessarily fix him immediately. If he picks up the wrong lead I don't make a big deal of it. Technically I asked for the canter, and he did it, he just gave me the wrong one. So I circle on the wrong lead (just so it's somewhat uncomfortable) and the go back down to a trot, set him back up, and ask again. When he picks up the correct one I make a big deal of praise and let him see the correct canter is actually easier.