He’s your brother, you’re allowed to straight up tell him he stinks. Better you than his peers. Tell him normal people shower once a day and brush their teeth twice daily. It’s possible he’s unaware of social norms. Does he have his own toiletries? Maybe buy him his own grooming kit, ‘male’ shampoo, body wash, deodorant. If you’re feeling generous get him a nice cologne. Once he gets a compliment from a girl about how nice he smells you’ll never have this problem again.
Terrible acne made me OCD about cleanliness for awhile. I was super self conscious.
After that subsided, facial hair/sweat were obvious. I wanted to be attractive so I took care of them.
It may be something he just has to learn for himself. My younger brother just turned 16 and my dad still has to harp at him to shower and brush his teeth. Hygiene isn’t always easy to pick up for people.