I also wonder if perhaps he got a foreign body in there somewhere. My horse had one of those, about pea sized, come up in the same location as Jodi's horse did. I noticed it one day when the farrier was there. I felt around on it and squeezed a bit and Dobe wasn't too bothered so I went ahead and squeezed on it a little harder. Sure enough, it popped like a pimple and there was a barb off of a sticker mixed in with what came out. I squeezed out everything I could get, much like you would do with a pimple, and it went away and never came back.
If you do that, though, you'll want to make sure and clean the area very well afterward with alcohol and/or antibacterial soap.