My last boyfriend lived in the same building, different dorm room as me. So after an argument, we'd usually slip notes or letters under each other's doors. For example, I made him an origami swastika with a poem (I don't want to offend anyone, and it didn't offend him, it was sort of an inside joke referencing an earlier conversation). And another time, my boyfriend found a piece of paper that kept score for a scrabble game we played (while cleaning his room), and he slipped it under my door and wrote something like "I know you're still mad at me, but when you're ready to talk to me, wanna play another game with me?" I really love handwritten notes, because they show more personal effort and thought, and unlike texts, you don't feel the need to respond right away if the other person isn't ready to talk right away. The notes should just be something simple, asking to do something fun together when the other person is ready.
If you're into hip-hop music, you could link this song (Mad by Ne-Yo) to him.
http://www.youtube.com/watch?v=LVzRUDwW2rg (skip to 50 seconds in) OR
http://www.youtube.com/watch?v=sAYuOc7_TKg
But that said, you had an argument, and you just can't wish it away either. He might be looking for a sincere apology, or he might want to talk about it more, or he might just need some time alone to think. So you should respect that too, if he doesn't respond right away. Good luck!