Difficult this one. I work with students so I can sympathise, a lot of them are selfish little sh1ts! From what I've read I think he's taking the piss and he knows it. But blood is thicker than water so mum will naturally side with the son. However, Your M8 is paying half the bills so he's entitled to have his say. I think they all need to try to agree some guidelines together, e.g. Girlfriend or mates not both, music of at a mutually agreed time. Splitting the chores up around the people in the house. If I can offer some hope it's the fact that there's a huge difference between freshers and final year students, majority of kids after the full University experience mature into well rounded adults. If only your M8 can hang in there.
Treat everything in life the way a dog would- if you can't eat it or foc it, forget it.