will my sons Apple watch work w/o being near my phone?

I just ordered an Apple Watch for my son. I clicked on add a new line and purchased the watch. Now I'm reading about it and I'm wondering if this watch is actually going to work if he's not close to me? I understand I have to use the family connection thing which is fine. But. Will it only connect to one phone? Will he be able to make phone calls to other people or only the phone it's connected to- as in mine? He spends a lot of time in a different city because his father and I share time, so I'm wondering will his watch (the phone part) work when he is there? And do I understand correctly that he will have his own phone number?