Online safety for kids

I'm asking this on behalf of my sister, Kristy, 35. Lately, she's been very suspicious about her 11 year old son who's been caught visiting porn sites. She has had talk about it with him and he says he did it 'out of curiosity' when his friends wrote these websites on his notebook.

She tried to explain him how dangerous these websites are but for the past couple of months he's been trying to visit them when no one's around. She's looking for some advice how parents handle these situations and if there are any tools available for online monitoring of kids.