Keyword Blocking App/software

hello

I have a very young son who has been checking out inappropriate sites. I blocked access to those sites by using hostfiles but just to be extra secure, I'd like to block some keywords also.

Can one suggest an app/software that I can use to block keywords so any page associated with them wont load?

I tried K9 and CutX free trial and was NOT impressed one bit with neither. I would rather use an app that's free but IF the product works well, I wouldn't mind paying a licensing fee for it.

Would appreciate any suggestions.

Mark.