Are you using your son's Apple ID? Go to Settings. What do you see at the top of the page? Is it your son's name and picture? If so, you need to tap on that, swipe to the bottom of the screen and tap Sign Out. You will be asked for his Apple ID passcode. Ask him to type it in. Then, sign in with your own Apple ID. If you don't have one, you'll need to create one.
Create and start using an Apple ID - Apple Support
The fact that your son was on his phone at the time you were setting yours up has nothing to do with this.