For a couple days, or however long it takes for it to go smoothly, forget the barrels. When you are riding around your arena, imagine a barrel and ride a circle around it, at the walk. Hurry his feet around the circle. Once he makes a change, release everything and let him walk straight. Then do it right away, repeating this exercise until he moves easily off your leg around those circles and stays on the circle. In between circles, it would be best not to walk straight for too long, just keep making circle after circle. But also make sure when you feel he's made an effort, to be quick to praise him but allowing him off the circle.Don't be too picky at first, if he goes, allow it and do it over. Eventually, you can work up to walking into the circle, trotting through it, and traveling straight again. Once that happens, re-approach the barrel again and do the same exercize. Start slow, though, and accept little offers he makes to please you. It sounds like he gets a bit stuck and doesnt fully understand the point of going around the barrel, so take a few steps backwards and re-establish his ability to hurry his feet around tighter circles.
Hope that made some sense, and good luck! Let us know it is goes!