The earliest thing I can find is an 1850 census record with him and Emma listed on it.
"United States Census, 1850," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MC1L-TBS : 9 November 2014), Russel Mathison, Painted Post, Steuben, New York, United States; citing family 695, NARA microfilm publication M432 (Washington, D.C.: National Archives and Records Administration, n.d.).
There's also this 1870 census:
"United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M8KF-N2F : 17 October 2014), Russell Mathewson, New York, United States; citing p. 102, family 846, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 552,594.
Try researching each of the children. That can sometimes help lead to unexpected discoveries.