I found Neil Simpson in the 1881 and the 1891 Canadian Census as one of the children of William and Helen Simpson. I was unable to find him in the 1901 so far. His mother is by now a widow, and listed as the head of the household, and she is left with the 3 youngest children at home.
To get an easy overview of all the Canada Census information, I added all the data in Canada Census Overview Charts
I did find Neil in the Public Archives in the Marriage Records.
||Cavendish, Lot 23
||Sarah Margaret McLeod
|Date of Marriage License or Bond
||11 February 1903
|Date of Marriage Ceremony
||16 March 1903
||RG19, Series3, Subseries4: Marriage Licenses, 1903
Strangely I did find him and his family twice in the 1911 Canadian Census in British Columbia, not sure the reason for it yet.
There also happens to be family photo at the PEI Archives.
Public Archives and Records Office
||Group – Family – Simpson (William John)
||1 b&w 35 mm negative
||ca. 1890; oversize; copy print in binder, Reading Room 
||identified are: John Clark Simpson, Winchester Simpson, Mary Ellen Clark Simpson, Lincoln Simpson, Jeannie Simpson, Neil Simpson, Charlotte Elizabeth Simpson, Ella Simpson
If any of the descendants of Neil Simpson want the book, with your great.,.,.grand father’s inscription, you are welcome to have it. It comes with handwritten notes for his diet..