« Reply #6 on: Friday 05 April 13 01:18 BST (UK) »
An Ernest S Johnson aged 40 died in Cambridge in Dec quarter 1947. That equates to a birth in 1907 before the date of death, or 1906 after the date of death.
That is the only good match for a death for the boy in the 1911 census (and the 1907 Smith birth if it is him). It might be worth getting the death certificate.
Ernest S Johnson married Edith C Robinson in Cambridge in Sep quarter 1935. That certificate might be more interesting than the death certificate. There don't seem to be any births to the marriage in Cambridgeshire - 1929 is too early, and 1951 twins seems a little late. Edith Clara Robinson was born 1910 in Chesterton district and Edith Clara Johnson born 1910 died in 1988 in Cambridge.
Assuming that Johnson is the given name that the "J" in the 1907 birth stands for, it is maybe more likely that the mother was Smith and the father was Johnson; that was a common naming pattern for children of unmarried parents: give the father's surname as a middle name.
HILL, HOARE, BOND, SIBLY, Cornwall (Devon); DENNIS, PAGE, WHITBREAD, Essex; BARNARD, CASTLE, PONTON, Wiltshire; SANKEY, HORNE, YOUNG, Kent; COWDELL, Bermondsey; COOPER, SMITH, FALLOWELL, WILLEY, Notts; CAMPION, CARTER, CRADDOCK, KENNY, Northants; LITTLER, CORNER, Leicestershire; RUSHLAND, Lincolnshire; MORRISON, Ireland; COLLINS, ?; ... MONCK?