Trying to set up blocking of steam and other sites without the person knowing Nighthawk R7000 Router

I have a 21 year old step son who has developmental disabilities and freaks out if we flat out block his internet in order to get him out of his room. I've tried to block the ports for Steam, but I don't think that's fully worked. Is there any way to block access to sites/services that don't explicitly say that the sites or services have been blocked by Netgear? Essentially I'm looking to keep his access on, but stop him from playing games online and accessing VPNs to bypass that during certain times of the day.