First thing to note: if your son is under 13 (in U.S.), he cannot have own Fitbit account so you will not be able to do this.
With the Blaze, any personal data is gone when you connect to new account.
To do the switch, first do a final sync with your account if you have any data you want to go to your account.
Then, on phone app, tap profile pic at top left to get to Account page, then tap Blaze, then tap the wastebasket icon at top right. Then, in phone settings, unpair the Blaze.
Then, in son's account, on Account page, do Set up a New Device.
Best AnswerUnfortunately, only the Ace or Ace 2 can be used in a family account.
Best Answer