Man up and just say something to him.
"Hey, bro, I thought I saw you looking at some questionable material on your computer the other day. You know, since we work around kids, doing that is unacceptable. How would you feel if your child stumbled on pictures like that. I know it would upset me. I would appreciate you using better judgment in the future. Thanks."
You can even print this post out and memorize it if you want.
You can do it!
I don't know what you saw, but considering some of the links posted on this board are not labeled "nsfw", maybe he just followed a link like that and ended up where he shouldn't have been.