Spooky horse...Any suggestions?

I bought a 16 yr old TB that was being used as a school horse for jumping. When I ride we will be walking along fine and he will spook and turn really fast and run. I know you should turn them back and get them to see what they are afraid of but my problem is I have no idea what is causing him to spook. So what I'm doing is going back to where is spooks and doing circles. Does anyone have any other suggestions?