I am assuming that your final sentence was going to say, "of same phone number?" If that is what you were asking, I'm 99% sure that the answer is "no." I'm doubtful that you can sync two watches to the same phone to begin with... but that's not the main obstacle. Even if you could, both watches would share the same phone number as the phone, and you can't call yourself; your call will take you to your voicemail box if you do.
For this to work, I believe you'd have to have a separate cell phone for your son, with its own number... and then the watch could make and receive calls independently even when the son's phone is somewhere else. Note that some functions depend upon the phone and watch being paired and connected to each other through Bluetooth.
Don't know about GPS tracking of his watch by your phone....