hi Elizabeth,
the 1900 census for the family shows him born May 1861, Canada (Eng). So not likely the last name was Simard at birth. There are Seymours in this province in the time period, but no Joseph found. The only approximation I have seen of the name Seymour was Simmeurs (written by someone who did not speak English). I checked 1859-1863 for any presence of Seymours in this province in IGD, since birth years are sometimes off. Quite a number of them present, in various regions. Unfortunately, IGD's indexation stops in 1861 for baptisms currently, not sure all the parishes are done either.
Since it shows him as migrating in 1865 to the USA, 1870, 1880 and 1890 censuses of the US might be the place to look for further data. I know, needle in a haystack, but I think you can limit searches by age. Other than that, his marriage needs to be found.
PS: Source #2 on Agnes's profile is a blank.