While it is never explicitly stated, there are several clues pointing to his last name being "Grayson".
The address of the Grayson Branch is: 1446 N. St. Hwy. 7, Grayson, 41143 6465
Grayson McCouch is 6'.
Grayson Kashino is 6'.
John and Mary Grayson
It's black and white, Not blue!!
Greyson Michael Chance
The name Grayson comes from an English surname meaning "son of the steward", derived from Middle English greyve"steward".
Seán Dolans GAC was created in 1942.
No, his original name is Grayson Chance.
Evan Grayson's birth name is Evan A. Grayson.
Grayson Berry's birth name is Grayson Frederick Berry.
Alexander Grayson's birth name is Alexander James Grayson.
Hal Grayson's birth name is Harold Wesley Grayson.
Connie Grayson Criswell's birth name is Constance Grayson.
Larry Grayson's birth name is William White.
Misty Grayson's birth name is Jane Burgess.