Usually when the blow an abscess you can find the drainage. It would make sense though if he bruised his hoof or had an abscess come on and then blew it, drained it, sealed it, and it came back (since he's been lame off and on).
That's kind of where the soaking with Epsom salt comes in. You want to keep it soft and open so that it can completely drain or it's going to become an on going issue. Ichthomal would be helpful in this instant as well, to help pull all the yucky stuff out.