Reddit, my roommate's IP address is blocked from 4chan for child porn -- how should I go about investigating this?

I apologize in advance if I use the incorrect terminology, or if my statements make me sound stupid as fuck. I'm not particularly technologically minded, and definitely not very well-versed in networking/ISPs/IPs/etc.

I moved in with my roommate a few months ago, and we agreed that we'd just use his internet connection. earlier today, I decided to finally check out 4chan again (I hadn't been there in a LONG time), and I got a "You are Banned" message (for "CP," which I'm guessing is child porn). it dates back to 13-Oct-2007.

my roommate's not home right now, and I don't know if the IP we're using was recycled or what. I'd like to investigate the situation, but I don't know what to do. I definitely don't want to accuse him of anything, and I don't even know if he uses 4chan.

tl;dr: using my roommate's internet, I found out 4chan blocked his IP for child porn. now what?