I have a young horse, turning 4 this spring, we broke him last summer as he was ready for some ground training and saddle sweat work. But then he had most of the winter off, just some trail riding. So for the last I would say 4 months, due to weather and daylight issues, he was only being ridden 2 times a week, and on the trail, as soon as we broke into canter, he'd buck my butt clear outa the saddle.
Great! Not the last bit of course!
But I think it's great that you've only just lightly started him. As the growth plates in their spine aren't 'closed' until around 5-6 years old, I don't like to hear about people doing too riding on immature 'kids'.
The bucking; have you checked your saddle recently? Saddles should be checked for fit around 6 monthly, but on young growing bodies I think it should be done at least twice as often. Have you ruled out any other source of pain, such as bit, teeth, feet, rider weight & balance, that may be causing this? If you've ruled out pain, do you think it's fear or something else?
So now that we are both sadly outa shape, and just starting back up, I'm finding he's running into his canter and landing on the wrong lead.
I'd personally forget about leads for now & just get him happily & reliably making canter transitions. Once he's learned to do it at all, then you can start 'refining' it with leads, etc. There are also the possibilities of your balance, the saddle, hoof form & other body issues that could be making lead changes difficult for him.
Now I'm thinking because he's so outa shape and so young, to hold him back and demand then he hit the canter immediately when asked is too much for him right now.
Agreed. Start at the start - or wherever he's reliably & happily up to - & move forward onto 'harder' things only after he's mastered the easy stuff.
At this point if I ask him to shorten his super long stride at the canter he can only hold it for a couple of steps and then breaks into a trot.
I'd say great - reinforce that!