A dormitory bulletin board provides several different opportunities to pull pranks. You can list your roommate's personal cell phone number on the board with "call for a good time" under it. Another option is to post photographs of him, using Photoshop to alter them and add either tattoos that he does not have or depict him wearing women's clothing.
Surprise your roommate by altering his toiletries. This could include taking his shampoo bottle and replacing the shampoo with another liquid, such as sun tan lotion. Or, replace his can of shaving cream with a bottle of whipped cream. These pranks will not injure the victim, but they will result in the desired surprise and shock.
Surprise your roommate by switching all of his clothing with another friend's items. Ideally, switch with a member of the opposite sex. This is best if he is a solid sleeper and you can do it while he is sleeping. That way, when he wakes up the next morning and is in a hurry to get to class, he'll be surprised to go to his closet and find that all he has are skirts and dresses to wear.
Shock your roommate with a little time warp. This is especially effective if you know he has an important test or presentation in class that morning. Change the time on the clocks in your room, and the time on his watch or phone, so they all match. The key is not to go overboard; make it believable by only changing the time by a couple of hours, keeping in mind that sunlight or lack of it could ruin your surprise. Your roommate will panic, thinking he slept through his exam or presentation.