Recently, my brother and his girlfriend came rip-roaring through town on a summer trip across the US. I was happy to provide them access to a comfy bed, laundry machines and a regular shower. My brother, who smelled musky, did not shower the first day, and on the second day, I told him that he smelled a little ripe and ought to shower (perhaps a bit blunt).
His response was that it was better for his skin to shower less frequently (he has always had bad acne), and his girlfriend liked his smell. They had researched why she would like his odor and there is definitely science behind it. I, however, did not find it appealing. I decided not to push the point, but after he left, the whole guest room smelled like him, even after we changed the sheets, for a few days. I find that a bit excessive.
How do I tell him to shower more frequently?
If there's anyone you should be able to easily tell "you stink!" it's a sibling. And, it sounds like you already did tell him as well as suggest a solution (a shower) and he rebuffed you. So maybe the question isn't how to tell him to shower more frequently, but whether you want to host him in the future.
This topic would be more awkward with a houseguest who wasn't also a close relative, but since he's your brother and seems aware of his odoriferousness (perhaps his ecosystem hasn't reached it's "steady state" yet?), keep being blunt if you really don't want to host him sans showering. Tell him you're happy to have him visit, but there's a "shower stipulation." Or, decide he's a grown-up making choices about his body that you don't like but can respect and buy a cheap set of sheets you only use when he visits.