He may well have an SI issue. May have had it prior to going to the trainer…which would account for a “fast” canter. I don’t know your trainer of course, but many of them will resort to mechanical means to get the horse to shorten their step if the goal is to slow the gait. This could certainly exacerbate a back (or pelvis or stifle or hock) issue.
If he were at my house I would probably turn him out for a while and let him be a horse. “Benign neglect” seems to fix a lot of issues. I have rehabbed many older “broken” horses and turnout almost always seems to solve problems like you are describing. When a horse can move freely, role when he wishes etc he can often “adjust” himself. Time is likely what you need here.