Hello everybody, 
All I seem to do is hit brick walls 
Molly
Hello Molly,
have you tried accessing any of his siblings' wills  on Scotland's People website?? Wills  may give some insights to siblings or nieces or nephews of John Nicol, grocer from South St Perth. 
I believe that the Robert Nicol, shipowner, born Arngask, Perthshire (now known as Arngask, Fife) who died in Perth at York Place  in 1865,   and the James Nicol, grocer, born 1785, Dron, Perthshire died of typhus, 1848, Perth city, Perth, were two of your John Nicol's siblings. 
 You could also try to find a way back by checking out John's other siblings - they may have left  source material that would verify that John Nicol, grocer from Perth was a sibling:
William Nicol, Dron, Perthshire and and Magdaline Moncreiff, Fortrose, Perthshire married  29 December 1783, Dron Perthshire Scotland. I believe you have previously posted a query about this couple's ancestry? DonM seems to have given a definitive answer  - that Magdalene was the daughter of Mathew of Culfargie which is NW of the town of Abernethy, where Magdaline was born..
Their children included:
James Nicol, bapt 27 May 1785,Dron, Perthhsire . Grocer in High Street, Perth. Died age 64  of typhus, 10 Mar 1848, Perth. Buried at Greyfriars, Perth 
John Nicol bapt   6 MAY 1787 • Dron Perthshire Scotland. Grocer at 178 South Street  Perth. Died 1849, Perth. Buried at Greyfriars, Perth 
Margaret Nicol, bapt 9 Jul 1789, Arngask, Perthshire, Scotland
Robert Nicol born 27 Nov 1791, Arngask, Perthshire, Scotland (Arngask is now considered to be in Fife) Death 1865 • 10 York Place Perth, Perthshire Scotland
  m 1: Christian Will (1794-1823)  2:Margaret McEwan  (1798-1872)
Wills on SP:
NICOL ROBERT 27/04/1865 SHIPOWNER IN PERTH, RESIDING AT YORK PLACE THERE   PERTH SHERIFF COURT SC49/31/79
NICOL MARGARET 17/08/1872 ALIAS MCEWAN, WIDOW OF ROBERT NICOL, RESIDING AT YORK PLACE IN PERTH   PERTH SHERIFF COURT SC49/31/94
William Nicol bapt 19 Jan 1794 Arngask, Perthshire. 
Alexander Nicol bapt 29 Nov 1795 Arngask, Perthshire
David Nicol, bapt 11 Feb 1798, Arngask, Perthshire
If you can research the siblings of your John Nicol, you may find on their death records, (if after 1855),  information about their parents. Obviously the father will be William Nicol, but the mother will also be named - they will be proven siblings of John Nicol and James Nicol  from Dron,  Perthshire if the Nicols born in Arngask have the parents  William Nicol and Magdaline Moncrieff.