Apple Watch 5

When you activate the iphone 6s make sure you choose a network provider that supports the apple watch,not all do,the cellular watch uses your phone's number to make and receive calls via an E-sim that is built into the watch.

You could also buy the GPS only version on the watch which will make and receive calls as long as the phone is with you,this will save you $10 a month if you're in the US plus you can use it with any network carrier and not have to worry about finding a carrier that supports the apple watch.

FYI: The Apple Watch add a monthly fee to the iPhone bill. In the U.S. it is $10 per month. So you are not getting 2 cellular lines, you are getting 1 line that is slightly more expensive than a regular line.

However, you want your son to actually have the iPhone with him (or at least in the house turned on), because the Apple Watch battery is not big, and radios draw lots of power. If the iPhone is nearby, the Low Energy Bluetooth will use the least battery and ask the iPhone to do the heavy lifting.

If you try to live totally off of the Apple Watch battery, it will run dry before the day is over.

No you can't do this because the cellular watch uses the same number as the phone,if there is no simcard in the phone then the watch cannot make or receive calls.