[Quiet Man Cometh]

There is not much else you can do.

Gotta pick your battles with kids, sometimes too. If the mess is in his room, let it be, it's his space. Ignore the stuff that is annoying but harmless. Save the nagging for when it's important or relates to you and your space and so he doesn't just start tuning out everything. You might have to start developing your own approach with your brother independent of your parents.