Teen making noise at night

My son has ADHD and opposition and provocation disorder. He’s speaking really loudly late in the evening in front of his computer and I’d like to set boundaries without being manipulative but the only thing I’m thinking about are things like “if you don’t stop shouting at night while you’re playing I’m gonna cut the internet connection” and it feels like threat.

So if he was an adult not living in my home, I could just decide to leave and not be around him when he’s making this noise which would allow him to do whatever he wants and me to have a boundary for myself but since he’s my 15 y.o son and living with me, I don’t understand how I can set boundaries without for him to change his behavior for which I thought was the difference between manipulation and boundary. Can you help me with this please ?