I read 11 yo Thorobred, ex racer for 10 years, 4 mths at new home with new amateur owner, becoming assertive,, doesn't like round pens.
So perhaps this TB - a breed known to have intelligence as well as speed,
Has been retired to a nice gentle home which he has now got know as being a soft touch.
He is in the prime of his life, he is used to racing fast against his mates in a straight line and now he is being asked to go round and round in small circles learning how to walk properly - when he believes he already knows how to walk and run properly.
JSB - have you asked yourself if he is bored, maybe overfed and certainly under worked - compared with his old life?
The cure? - that's more difficult to suggest - perhaps he still wants to race in the Pensioner's Derby?
Over here many such ex race horses are put to steeple chasing or hunting or team chasing. Such sports they are good at.