After going over his feet, poking and prodding, tapping and trotting, no real source of his pain could be found. His right fore does have some "banging" in the digital pulse though, after I knew where to look I could feel it. That really only leaves laminitis as the possible culprit.
We think we know what might be causing it, his lameness correlates with the delivery of freshly mown hay. It might just be too rich for him. I'm getting him cut back to half rations to see if he starts feeling any better.
Please do the trick, please do the trick, please do the trick