The five Nobel Prizes are awarded for outstanding contributions in the fields of physics, chemistry, physiology or medicine, literature, and work for peace.
While obviously they can't give him a Nobel per se in those official five fields, if they would ever give him one it would be probably be the 'Nobel Memorial Prize in Economic Sciences' which is awarded by the same people who award the Nobels for Chem and Phys. But, that is assuming they're willing to give it to someone who is considered to be anonymous...