our kids are still fairly young, but of all our friends with kids, regardless of age it's pretty much the general rule that kids don't have computers in their rooms, even if it's borrowing a laptop. i'm not sure if when you said 'i walked in...' you meant to his room, or if he was looking that stuff up at the kitchen table. our experience has also been that less is more in the information department. sometimes we as adults think our kids need lengthy discussions about stuff, maybe he was just curious what naked women look like, and he should have been shown a text book!
i only say this because of a funny story about a boy who asked his mom where he came from and she went into a big story about 'birds and bees' and how 'when two people love eachother...' when she was all done she said, 'does that answer your question?' he just looked at her and said 'well i just wondered, since jimmy said he came from ohio'
on another note, i think your husband really needs to be the one talking to him about this. (not that you can't have any input, especially since it's YOUR computer) but any boy, especially 9 yo ones, are going to be embarrassed beyond comprehension when their mom catches them looking up naked ladies online. he'd probably say ANYTHING to stop talking about it. most boys will be much more able to actually have a conversation about sex/women/guy stuff with their dad. (sure there are exceptions, but they're rare)
i don't know if this helps, but i hope you get it worked out!