He needs to shower more and change and wash his clothes (and bedsheets) more often, the mustiness comes from a continuous lack of keeping himself and his clothes and room fresh and clean, he definitely also needs to keep his window open more and air out his room.
My ex rarely wore deodorant and if he did it was usually natural, but he showered at least once a day and wore items of clothes for max 2 days before washing them, T-shirts usually only one day. Despite being a superrrrr active person he rarely smelt of BO or anything bad (and anytime he did I told him to shower lol). So that’s not necessarily the issue.
Definitely say something, I used to live with a guy who’s exactly like you described, he and his room and his clothes always smelt bad and really musty and you could smell his room from a few metres away with the door closed 🥴 so I completely sympathise that sh!t is nottttt pleasant 😭 do not suffer in silence!!!!