Why does my 8yo ex-racehorse refuse to run?

His former owner said that he was a winner- but one day out of the gate he refused to run. He was then retired- and has ponied horses on the track. I purchased him and and can't get him to do much of anything and am very frustrated. He sands nicely to be mounted, but from there is slow and sometimes just stops. He is completely sound, has never been injured, and never had bowed tendons. I have hope for him and wonder if he is somehow not feeling at his best and am wondering if there is a way I can bring out some life in him. He is only a pleasure/trail horse- I am not looking to ask that much of him.