Apple Watches on one iPhone

Hi guys, apologies if this has been asked before!! I own AW4 and IPhone XR and want to buy my son a GPS cellular Apple refurbished AW3 on a separate sim on my current network.

I understand that I can pair his watch to my XR to set up but I don’t want it to be connected to my phone once set up in case he gets messages or calls instead of me( if that’s possible) and need to use auto switch to use my own watch.

is it possible to set up his watch and just let it do it’s own thing disconnected from my iPhone?

Thanks for your time, J