If you have some intermitant lameness issues it could have turned into an abcess. Maybe it is working it's way out. Sometimes they take awhile and can cause intermitant lameness, until they really get to the surface and then they are really lame. Maybe try soaking in some epsome salts and warm water and see if it gets any worse, this will draw it out and cause him more discomfort, but should bring it to the surface so it can blow, then he will be sound again. Good luck, hope you find the problem!