Windows 8 Making parental controls permanent

I have a 15 year old son. He was on his laptop as an administrator but was accessing websites that were not allowed. I went in and added myself as an administrator and then changed his account to "standard user" so that I could then set the parental controls. Apparently, however, he was able to go in to the control panel and simply switch himself back to an administrator. Is there a way to make it so that he cannot change himself to an administrator? If not this is a serious flaw in the software, since most 15 year olds know the OS better than us older folks.