My autistic step-brother talks loudly to himself.

I (f21) still live with my mum. She met a guy 5 years ago and he has an autistic son. His son comes to our house on weekends and his room is right next to mine. He's constantly in his room at the weekend, and whilst he plays roblox and watches YouTube videos, he talks loudly to himself. All day long, he talks loudly to himself, sometimes even shouting and stomping.

I work nights at a warehouse, so i usually sleep during the day but lately I haven't been able to sleep because of him. I understand that he is stimming, I also have an autistic younger sister so I'm used to stimming, but it is constant and loud. I have talked to both my mum and his dad but they don't ask him to be quieter. Even when I ask him to be quieter, he forgets after a while and its getting to the point where I'm exhausted and frustrated because of it.

Please give me some tips on how I can politely ask him to shut up so I can sleep..