If your son is anything like mine, he'll find a way to get a friend's phone to use for whatever nefarious purposes you are hoping to prevent.
FWIW, I have a Slim Devices squeezebox that I'd part with for a few $. It's from before Logitech bought the company. I believe it's the first SB I bought, probably 25 years ago. I don't have the remote control, but that's probably pretty cheap via ebay. I do have the original manual. Something changed in WiFi about 6 or 7 years ago that keeps old squeezeboxes from maintaining the wifi connection with modern routers. Someone wrote a fix (see the LMS forums) but I never got it to work. If you connect via ethernet cable it works fine.
You could put the LMS server in the kid's room, and don't connect it to the internet. Add a $90 SMSL SU-1 DAC and he can play files using SqueezeLite or directly from the HDD.