Hey unfortunately ex racehorses commonly have this problem as the gain more muscle on the side the were raced on. i agree with getting the physio to have a look. try over exaggerating your transition and opening the inside reign a little bit, asking on a circle will help as well. try and ignore the bucks as this might be his way of saying he doesn't know what your asking or is in pain. best of luck. racehorses take a lot of time and patients so remember that when schooling.
Hey Sofia, this is very common with the ex racers. I have worked with two that had this issue. I would suggest that as long as he is not in any pain( as a lot of ex racers can have health problems) then he is probably unbalanced. I would work a lot of your trot and small bursts into canter, just be patient with him and give him time to understand what you are wanting. He might also benefit from some lunging sessions.
He is probably just really unbalanced so you can try counter flexing him in the trot and rising on the wrong diagonal and push him into canter. I know it sounds stupid but give it a try 😊. He might also be stiff in his back, have you had a physio out to look at him?