This sounds like sports reporters gossiping around the water cooler.
There have been many instances of nepotism in the NFL's coaching ranks, but never has a son directly replaced his father as head coach, and the Patriots won't be the first. Even if this is true and Belichick wants this, hiring his own son as head coach would rightly fall into the very small category of things that Kraft would/could veto.
Having Bill in the Parcells puppet master role casting a sizable shadow over his own son sounds like an overly messy situation on many levels. Every son of a successful NFL coach has eventually had to move on and make their own history with other teams, and It won't be any different for Bill's sons.