Sweet potatoes and white potatoes and yams are tubers. Just buy him some of those? Close enough?

Art

You are correct that they are all tubers, but sweet potatoes aren't related to the others. More a large vine than a small potato plant.

from wiki: The sweet potato (Ipomoea batatas) is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are an important root vegetable.[1][2] The young leaves and shoots are sometimes eaten as greens. Of the approximately 50 genera and more than 1,000 species of Convolvulaceae, I. batatas is the only crop plant of major importance—some others are used locally, but many are actually poisonous. The sweet potato is only distantly related to the potato (Solanum tuberosum).

http://en.wikipedia.org/wiki/Sweet_potato