In reply to Scarab9:
This rings a few bells from my student days... We didn't manage to find a solution which everyone was happy with. I think we got a token payment which wasn't enough to satisfy the person who felt the extra lass should contribute, but was enough to sour our already strained friendship with them.
Is it a HMO property? If so, and it's licensed for three tenants, adding a fourth could cause legal problems and a large fine for your landlord. Followed potentially by evictions and bad references, if your landlord thinks you've breached your contract. How the law separates a guy staying over with his girlfriend fairly often from an extra tenant I have no idea, but I don't think him paying rent would help your case.
That said, I do think, morally, that he owes you a fraction of the bills and maybe some rent money. Perhaps a good approach would be to adjust the rents to accurately reflect room size, and he can help his GF make up the difference. Plus some cash toward bills - cooking and showering both use a lot of gas/leccy and will be adding a good chunk to your bills.
It also sounds like he and his GF need to have a proper conversation about their relationship, where it is and where it's going. But I doubt you can help with that.
Post edited at 18:28