You two probably won't have the same class schedule, so jerk off when you know he's in class. As for showing off your size, you're going to see each other naked; it's inevitable in such situations with changing and showering. Don't try to cover it up or timing it to when he's not looking - it just makes your life more complicated and, when he finally does happen to see it (and he will), it will be a big deal whereas, were he to see it the first day, it's old news in a couple days.
Another thing to consider is that he's going to need to jerk off too, so be aware of what time he does it, how he feels about it, and you'll know what he's comfortable with. The worst thing you can do is pretend that you don't jerk off and that it's "disgusting" that you're roommate does, cause that sort of attitude will blow up in your face (no pun intended).