You know.. he might not FULLY understand that you're asking for a "canter".. his interpretation of it may be that you're just asking faster, faster, faster.. and when he does what he knows.. faster faster faster.. it isn't good enough...
And he gets confused and starts trying to avoid it and getting pissy.
Some horses are a little more mentally "slow" than others. And "ask, tell, demand" doesn't work as well with a horse that isn't real clear on what you're asking, telling, demanding... it will just lead to sourness, evasiveness, and a lot of bad rides.

He's also an Appy and at an age that puts him on the same par as a mid-teenager human... So, you're going to get some snot-nosedness, arguments and bullheadedness.


If you're riding him with someone else... if they transition to a canter, does he willingly, or is it the same issue?