If he gets the lead on the lounge line, then the only things that change is that there is a saddle, bit. And a rider on him. If you take a look at the Turning Tighter topic I explained how to make sure your saddle is sitting in the correct spot on the horse to allow the shoulder maximum freedom of movement. It's different then what you might have heard. The saddle being too far forward could be your problem.
Next, check your position. Maybe you are unconciously throwing him off balance. Remember to sit BACK when you ask for the canter. Don't lean forward or drive him into the canter, that will only throw your weight onto the forehand. Sit back and "canter" in your body, just like he has to do in his body. The horse's weight distribution in the canter is 70% on the hind quarters, so your weight needs to be back as well.
Also, his teeth might not be balanced. If the teeth are not balanced, then the whole horse will not be balanced. You can check to see if he is balanced by looking at his eyes. If they are on the same plane, then it's likely they are balanced. If one eye appears droopy or lower, then his teeth are not balanced. Also, check to see if his cheeck bones are even. A lot of horses are asymetrical which indicates unbalanced teeth. I would HIGHLY suggest having one of Spencer LaFlure's certified dentists come out and look at your horse. Spencer has a different approach to floating teeth and no one else will ever float my horse's teeth. The results I've gotten, along with our friends, have been incredible and immediate. His website is www.advancedwholehorse.com