What modem/router do you currently have? Do you have an extender? What does a speedtest show you? The Actionec is rather limited on advanced features, as an example QoS.
I bridge my router and use my own router for more features and control of my network.
Steam has the ability to limit the bandwidth used right in the settings. And you can limit the bandwidth on Netflix as well. Does your son have a 4K monitor? If yes Netflix or even YouTube, for eligible videos, may try push 4K content which takes more bandwidth.
For adjusting data used for Netflix, see instructions on their website. Netflix's HD streams are only 3mbps so they don't use much by default.