Was he born in the IOM?
These are the ones in the 1911 census:
Name Birth Year Age Se x Registration District County Household Transcript Original census image

Household TENNANT, Sidney 1903 8 M Camberwell London VIEW VIEW
Household TENNANT, Sidney 1904 7 M Cleobury Mortimer Shropshire VIEW VIEW
Household TENNANT, Sidney A 1903 8 M St Pancras London VIEW VIEW
Household TENNANT, Sidney Granville 1906 5 M North Bierley Yorkshire (West riding) VIEW VIEW
Household TINNANT, Sidney 1904 7 M Brentford Middlesex VIEW VIEW

Sue