@JR
Or he can take each sub-niche and create a category listing for each, and as he does posts, he just checks off the categories the current post fits into. That way readers can just go to the specific sub niche that interests them. He should also try to use the sub fetish terms in his post for SEO.
(ooops, soorrry Ben, I didn’t see your post til after I published my reply~!) GMTA