One online genealogy has his name as William S. Tefft (needs more research). According to information for their sons Asa and Nathan in the 1880 Federal Census, both of their parents (Nathan & Sarah) were born in RI.
The History of Washington County, NY (Google Books) has information on this family, pp 291 and 337. It says that in 1766, Judge Nathan Tefft came from RI to Washington County with his two sons, Nathan and Stanton; more details.
An online family tree at Rootsweb's Worldconnect refers to a Tefft genealogy that says this Nathan was a son of Stanton, who was a son of Nathan Tefft of RI.