How do I get my brother to start showering regularly?

I (25F) recently moved back in with my parents and younger brother (23M) following a few personal troubles which means I’m stopping here for a while. It’s not permanent as I’m off to university soon.

My brother’s personal hygiene has always been a bit off, but it’s gotten immeasurably worse since he ran into a few mental health problems a couple years ago. To be clear, I’m not looking for any advice in regard to his mental health (although I’m acutely aware that his health problems do contribute to this issue).

We recently moved house following 12 years in the last one. We were clear to him about it being a fresh start, as his room in our old house (which was mine when we were younger, but he moved into it when I moved out) was a cesspit, to put it nicely. Stacks of unwashed plates and glasses and a smell I’d compare to a dead body. It was unrecognisable to me.

We’ve been in our new home for about a week. He has yet to shower here, and his new room is already starting to smell. It’s gotten to the point where my parents are asking me to get him to clean himself, because sometimes he actually listens to me, unlike them.

I sat him down today and had a very difficult conversation where I told him that it’s completely unacceptable for him not to shower as an adult man and that he frankly stinks. He was quite unreceptive. I’ve coddled him for months and I thought it was time to just lay on the cold, hard facts.

I don’t know what to do anymore. It’s affecting the rest of us in the house. I was wondering if anyone else might have some advice regarding this. Thanks all.