Forcefully explain that he's not your boss.

"Listen here, you little shit," you growl. As Terence's eyes widen, you cross the room and grab him by his stupid collar, lifting him off his feet slightly. "You don't make the rules here, got it? Do you live here alone?"

"I - I - "

"Answer the question!" you bark.

"No!" he shrieks.

"Oh, really? Then this is my room too?" you ask pleasantly.

Desperately, he nods.

"Then I'll do what I want in my own fucking room, Terence," you growl. You release him and he darts away from you, cowering. You ignore him and go back to unpacking. "And if what I want to do involves having the occasional drink...or the occasional lady friend over...that will be ok, because it's my room, understood? If you really want I'll hang a sock on the door if a girl and I are getting busy, then you don't have to see or even know anything about it."

"B-but where will I go while you're in here doing...that?" Terence practically wails.

"I don't care! Go to a friend's. Or find your own girl who'll take you in." You can feel him shudder without even looking at him. "Or go sleep in the chapel, closer to Jesus! But you don't get to dictate the rules in our room. Got it?"

"...Got it." Terence sounds sullen, and for a few minutes you just unpack books in silence. You hear him get up and leave silently, and you let your breath out. Well, that was...kind of nasty. But still, you can't let this weirdo walk all over you on the first day...

Suddenly you realize that the dorm is weirdly quiet. Shit, are you supposed to be somewhere? Leaping off your bed, you scrabble in your already messy desk drawers and find your orientation schedule. You groan as you realize that the mandatory welcome-to-campus speeches are starting in just five minutes...about six minutes away. Terence no doubt was heading there, and he (kind of understandably) didn't remind you. Grumbling to yourself, you stuff the schedule in your pocket and leave the dorm at a jog, headed for the orientation.