Thanks--I bought him a pair of splint boots for his rear a couple of days ago thinking that's what it was--so either the area became warm because of the boots or because of the work. I guess I'll have to canter him to check now, lol. I've seen ice boots like this EZ Ice Universal Leg Wrap - Leg Therapy
Would that work or would he need something more like a cold pressure wrap?
The only thing that really clued me in on it in the first place was the fact that as soon as I stopped lunging him, he would rest his right hoof. He could bear weight on it, etc. when I asked him to move, but he would just put it up as soon as he stopped working every time.