O dear yes its very hard to block that sort of stuff now, i know..
6 years old is very young so i for sure i understand why you would want to, i have a 6 yr old nefu and whenever im staying/looking after him im so so very careful with things like that on the net..
Just a few suggestions, that ive learnt..
A pop up blocker for sure(is a must, speesh if someone on your comp looks at porn or adults etc..im not saying you do or anyone but just saying) But even that you need to be careful at what address(URLs) you go to, even through google, as a normal site(you would think) could have p0rn popups, as ive found at time(but it is a little rare)
Be in the room ALL the time also, even if your son is 6, belive me they do very easyly come across the wrong sites and places on the net
And as the other have said netnannys and blockers of sorts..
Acuatally im thinking, if someone or you no if its possible, Cant you some how adjust the setting with your sites you go to, or something like that?
Well hope that helps