would love to see a video

without seeing how he works, I'm thinkin' he's not reading you, or you're not reading him. or a combination of both. Did he behave in a similar manner before you started working together? Could be an old bad habit long in the making. Persistence, consistency, patience. I'm not a fan of swatting for any reason. All it accomplishes is making a horse fearful. You want to reward him when he responds as you want hin to, not by using force.

Maybe a good shortcut in the training process would be for you to pony him with a partner who will canter? Meaning you lead him on a horse who will, so he'll be very clear on what he's being asked to do. Or maybe even better, have someone pony him with you on him, and have your partner canter. It seems there's a miscommunication somewhere, maybe that will get it across to him what he's being asked.

The other thing that I thought of probably goes without saying. But I'll ask anyway. Are you absolutely certain there's no physical limitation that is preventing him from wanting to canter? If so, it wouldn't necessarily prevent him from cantering with his pals. He probably wouldn't let on if he were slightly impaired, while in the company of other horses. That ego thing, you know. It might be worthwhile to have him thoroughly checked out, make sure those feet are in perfect shape, and that there's nothing anywhere that would cause discomfort.

it'll help a lot to get a video of all angles