Help! Need a free web filter

Recently, I found my 10-year-old son was very strange. Many times when he was surfing the internet, I came to him, he immediately closed the websites he browsed. I was worried that he looked at some bad websites. So I need a web filter, which can be used as parental control and block unwanted websites and applications, to restrict my son to looking at those bad websites. One of my friend recommand Aobo Porn Blocker,

http://www.nabalkone.com/sexvideochat/ 

but it's not free, I don't know how to choose. Can anyone give me some advice? Please help, thanks a lot.