Cold hose him! Or atleast some kind of cold treatment, i would cold hose 3 times daily for 20 minutes if possible then bandage at other times. Obviously don't work him for awhile. Is he very lame? If only slightly you could get away with just cold hosing bandaging and rest but if its bad lameness then you should call the vet to give painkillers. It may be the hard ground has made him sore.
I know wretched thing!!! and the worst of it is he doesnt normally do it, he just stands eating all day and looks at the others in disgust when they start runnning around
I've recently had tendon problems with my horse. Although mine wasn't lame he just had fluid along his tendons. The vet advised using a gel ice pack as the cold penetrates deeper into the leg than just cold hosing. I don't know if they do special "horse" ones but I got mine from Boots for about £6 and it has worked wonders.
Also I would suggest keeping him in if possible until the vet has seen him.