Can you get scurvy in two weeks? I'm thinking probably not, and when the this phase is done, he's supposed to transition to a vegan diet.

I'm guessing that the potatoes-only period is to make it more likely that the new diet will stick. A cleansing of the old eating habits before establishing the new ones.

EDIT: Also, potatoes have a lot of vitamin C in them apparently, however cooking does reduce the content. Does eating an ungodly amount make up for the cooking loss?

In addition, many vitamins are fat-soluble (A, D, E, K), eliminating oil completely is not as great as it sounds.

Potatoes are rich in vitamin C, there is no risk of scurvy.