http://www.espn.com/nfl/story/_/id/18121942/nfl-2016-dak-prescott-historically-great-rookie-far
More facts:
To remedy the problem of different passing environments across eras, we can compare each quarterback's AYPA with the league average for that season. For example, Prescott's AYPA is 9.2 so far this season, while the league average is currently 7.2. That means Prescott exceeds the league average by a net of 2.0 AYPA.
Only three other rookie quarterbacks in NFL history with as many starts as Prescott have exceeded the league average in AYPA for their respective seasons by more than that. Otto Graham blew the doors off the rest of the league in 1946, averaging 11.2 AYPA when the rest of the league passed at a measly 3.7 AYPA rate. Y.A. Tittle averaged 8.4 AYPA as a rookie in 1948, compared to a league average of 4.6 AYPA. The third quarterback to top Prescott on the list is lesser-known Greg Cook of the Bengals, whose 1969 season beat the league average by 2.6 AYPA. Cook's career was cut tragically short by a chronic shoulder injury.
In short, no one has exceeded Prescott's performance thus far since before the merger.