They would have been married before government registration began. Church records are a possible source.

Unfortunately his death registration doesn't list his parents.

Henry Ferguson, died 9 Jun 1891, 84 yrs, farmer, b Prince Edward, died of dyspepsia/3 wks, Methodist, Chatham Tsp, Kent County, Registration # 007452.

This is a bit of a long shot as Ferguson is a fairly common name.
This is from Athol Prince Edward County 1851 Agricultural Census.
Henry Ferguson, Concenssion 1, Part lot of 19
Roger/Rogell Ferguson, Concesstion, Part lot of 19
Israel Ferguson, Concession 1, part lot 26

This seems to be the only part of the 1851 census that survived for that area. roger


Found this but there really isn't much information on it. Same area of Ontario as Prince Edward County. Both were hives of UEL.
Could be father?

Lt. Israel Ferguson
Event: Living
Year: 1786
Province: Ontario
Place: Fredericksburgh Township
Comments: Attached to late Kings Rangers Regt.
Source: Norman K. Crowder, Early Ontario Settlers, A Source Book, Genealogical Publishing Co. Inc., Baltimore, Maryland, 1993.
Volume_Page: 169
Note: The province and county are associated with the location of the record source and in some cases may not be the same as the place where the event occured.

Israel Ferguson
Event: Living
Year: 1786
Province: Ontario
Comments: United Empire Loyalist.
Source: The Centennial of the Settlement of Upper Canada by the United Empire Loyalists 1784-1884. (The Celebrations at Adolphustown, Toronto and Niagara, with an Appendix containing a copy of the United Empire List, preserved in the Crown Lands Department at Tor
Volume_Page: 173