I also use the TP-Link powerline adapters, and recently upgraded from the AV500 to the AV1200 units. These provided a big improvement -- but, at least in my house, still exhibited some quirky behavior. I saw vastly different speeds in each direction, as the units transmit and receive on different frequencies. This is why I ended up putting the Tablo on a 50 ft run of CAT 6 cable.
I also saw a big difference in performance by setting the QOS setting in each of the TP-Link adapters to VIDEO. Before changing the setting, a relatively small download (3-5 megabyte file) would send the Roku units into "Loading, please wait". Since changing the setting, the video has been hiccup free. I still use the TP-Link powerline adapters to connect to the Roku and the desktop, as well as feeding the other access points in the house.
Can your son try to access from another device other than the X-Box? I use the X-Box app on the Win 10 machine, and have not seen any problems with it, but have not tried it away from home. (I have used the android app for both phone and tablet use away from home; my daughter watches via her IPad from two states away.)
Let me know if your son can directly connect with Plex. He MAY need to log in to your Plex account in order to have access to the Plex Channels option. I do not know if Channel "rights" can be delegated to the users you invite to share your Plex.
Hope this info helps.