Hi again
For ref: In 1881, John & Agnes are in Cupar with their 2 sons:
John 15, Farm boy, b Cupar
David 11, Scholar, b Logie
John jnr's birth is on the IGI as 27 Nov 1865 Cupar
David's birth is on the IGI as 10 Sep 1869, Logie.
In Census 1891, John jnr is boarding with his married brother William at Rathillet Cottage, Kilmany.
In Census 1891, David is probably the one working as a farm servant in Leuchers Fife.
In Census 1901, John jnr is / probably a ploughman in Kettle Fife, married with 2 children
I can dee William & Boswell in the Fife Census: 1841 (Letham, Monimail), 1851 (Star Dalginch, Kennoway), 1861 (Dunfermline). 1871 (Dunbog)
And their son John, aside from the 1871 & 1891 you mention, in Fife Census: 1841 and 1851 with parents, 1861 as a servant (Leuchers), 1881 (Cupar), 1901 (St Andrews &St Leonards).
Happy to post up transcripts of any these Census if you should require them, just let us know. The 1841 though, can be found transcribed here:
http://freecen.rootsweb.com/cgi/search.plCheers
AMBLY