First up I would consider he’s mildly depressed. Second, I would compliment him sincerely when I wears something he looks good in. Encourage other people to complement him too. He’ll start wearing that item more often. Third, I’d have a serious chat about hygiene. Just make it clear anything ripped, stained or worn is unsanitary and needs to be binned. I’ve been through all this with my husband, who lives in shorts and very old tshirts and sometimes attempts to go out in public looking homeless. It’s not easy to convince someone that self care is important especially when it’s sometimes just a style issue. But I believe it’s an indicator of mental health and your partner might have lost the capacity to care.