Well, it may not be Ivan but I think it is. At the very least, I think the first letter looks exactly like the I in Ireland below. I realize Wm is often used for William but it doesn't look like that to me.
there is a family tree online, on familysearch.org, info by slightizer2768157, which has William I Spence, May 1805 County Antrim, Ireland, Death 7 March 1890 Arnprior, ON Burial Arnprior, ON (Albert Street Cemetery). I cannot say if it is correct or not.
I can't think that William Sr. would have left his youngest behind. I have been looking into the family more the last few days, and Wm. Jr. did live in Belfast when children Ellen, Ezekiel David and Margaret Jane were born. I don't know if William Jr. had a middle name.
Peggy