He's portrayed as an autistic savant, so I don't find it entirely implausible. There are documented cases of autists showing extremely impressive performances in arithemetic and memorisation, so why not subitizing too?
"Laurence Kim Peek (November 11, 1951 – December 19, 2009) was an American savant. Known as a "megasavant", he had an exceptional memory, but he also experienced social difficulties, possibly resulting from a developmental disability related to congenital brain abnormalities. He was the inspiration for the autistic savant character Raymond Babbitt in the movie Rain Man."
