Hi @Beat Michigan.
Welcome to the Sonos community and thanks for reaching out to us. I understand that you are looking for answers on how to integrate a turntable with your Sonos system. Let me help you out with that.
The Sonos Port, Sonos Amp, and the Sonos Five both have a line-in port feature that would allow you to integrate your turntable into your Sonos system. However, the Sonos Port and Sonos Amp use an RCA connection while the Sonos Five uses a 3.5mm headphone jack as a line-in connection (same as your headset or earphone jack/connector).
The most ideal product you would need to connect the turntable to Sonos would be the Sonos Five based on your preferred options. as the turntable uses an RCA connection that would need an RCA to 3.5MM headphone jack converter and then using the group and ungroup feature on the Sonos app, your son can group the Sonos Five with the Sonos device through the Sonos App to have the audio come out of both Sonos devices.
Let me share with you a topic here in the community that discussed more details about integrating a turntable to Sonos.
I hope this helps.
Please let us know if you still have further questions or concerns. I’ll be more than happy to help you out.
Thanks,