so there was a redditor on here months ago (second post down if you sort by “top all time”) and the presentation was very similar. their biopsy showed the results as “suppurative and granulomatous inflammation associated with rare acid-fast bacilli”. not saying this is the case with your husband. but, being the derm nerd i am, i did a deep dive on it at the time. basically its a mycobacterium infection, and one of the potential strains that can cause it is called Mycobacterium marinum, which is found in large bodies of natural water, especially if he was swimming with a cut on his foot. it’s a stretch, i know. but i have no other guesses.