Hoping for some help

Okay, so at work, me and my partner traded horses. She hated Jack, and no one else likes to ride him because of his attitude. She now rides Mickey, and I now ride the cranky big sorrel gelding named Jack. I ride him every morning before rides, and at lunch before the afternoon rides. Now I love him to bits, he's just amazing to ride, even with his cranky attitude (he doesnt like his job, when we are alone, he's amazing.. he also DESPISES MICKEY) He has ONE problem that I want to fix. And its this: He cannot for the life of him get the right leads at a canter. Ever, and its driving me crazy. I've tried changing directions, but he just doesnt get it. Now, remember, he is a work horse so I can't spend hours and hours of time on him, and I get only about 30 minutes in between rides twice a day, maybe three times if its not to hot to work him, to work with him (mostly to keep him from getting bored and ripping apart his stall)

Does anyone have any suggestions? He's a very smart horse (he uses a comb to gather left over oats in his feed trough in his stall! He taught himself)

Id really like any advice. He's a lovely horse, but when going on the wrong lead... makes for a very sore bum!

Thanks guys!

PS.. lol fun fact: I have to be all spider monkey to get up on him haha. He's very tall, and Im very short. Im happy he stands for me. He used to take off with Molly (his old rider) all the time, so I was kinda worried to ride him, but he's an angel for me. Love him haha.

I saw a clinic once that addressed this exact thing. What the instructor had the rider do was ride one end of the arena on the left lead, halfway down the long side, drop to trot and make a turn to go across the middle of the ring. Trot all the way across to the end. The last stride before the wall, ask for the right lead canter. Make sure to raise the right rein a bit to free up his right shoulder and overexaggerate your cue for right lead. Even if he picks up the left, continue around the other end of the ring and then drop to trot and come across the middle again. This time ask for left lead at the end. Next time right, then left, always change it. The rider at the clinic did it about 6 times before the horse picked it up. If you do get it, let him canter until you think it might fall apart and then ask for the trot on your terms. Good luck.