Hi,

I found the following family on the 1881 British census on-line at www.familysearch.org.
I don't know if these are the ones you are looking for, but the age, and the location of the John R. McKay listed is about right for the John McKay in your query.

William H. MCKAY-head-M-male-23-Rushen, IOM, England-general laborer
Catherine J. MCKAY- wife--M-female-24-Lonan, IOM, England
Lizzie MCKAY-daughter-U-female---6-Douglas, IOM, England-scholar
JOHN R. MCKAY-son-U-male-4-Douglas, IOM, England-scholar

Dwelling--14 Queen Street
Census place-Onchan, Isle of Man

Also, there is a John McKay listed in the 1901 UK census.

John McKay, 23, born Douglas, IOM, civil parish- Onchan, occupation-sailor. (seems to be the right one. This John Mckay would be 73 yrs. old in 1951)

There is a fee to get more info off of the 1901 census, but if SKS on the IOM has access to it they might be able to help you.
Hope this helps.