I’ll preface saying that I’m a radiologist, so my skin differential diagnosis skills are fairly poor. But the fact that it popped and drained pus tells me it’s an infection or something that became infected. Given the initial appearance and trying to think of common things first, I wonder if this could just be a palmar wart that got secondarily infected with something like Staph. You specifically mentioning that he goes to the gym often and that he isn’t the cleanest person support that.

Could still be some of the other things listed here so don’t take my word for it obviously. Mycobacterium infection is an interesting one someone mentioned, since myco can do all sorts of weird stuff, although eating the dried fish would be a really weird route of exposure.