Many trees on A****y want John Grant to be the son of Peter Grant and Jane (McPherson) Grant. I found this family in the 1851 census and began trying to find them in 1861 with the aim of possibly ruling out this John Grant as being the PC in Glasgow. I didn’t find them.
The A***y trees maintain that Peter Grant died in 1851 and Jane Grant died in 1858. I haven’t found the original tree (with documentation?) that everyone is copying from, and I have failed to find either of these deaths at SP.
Jane Grant, widow, 52, parents John McPherson and Christina McLean, died at Rymore, Abernethy and Kincardine on 24 October 1858. Informant was Charles Grant, son.
At Rymore in 1851 are Peter Grant, 48; Jane, 44: Charles, 21; Christina, 18; John, 13; Peter, 10; William, 9; Lewis, 4; Gillies, 0. Christina, John, Peter, William and Lewis match the baptisms of the family of Peter G and Jane McP.
In 1841, at Cullichie, Abernethy are Peter, 35; Jane, 30; James, 10; Christina, 8; John, 3; Peter, 0.
So the Anc* tree does seem to have got this right, and Peter senior must have died between the 1851 census (31 March 1851) and 31 December 1854 (the day before the start of civil registration). There is no likely death on SP, so it may be that there is a gravestone with the date of death of Peter senior.
So it is possible, perhaps even likely, that this is your John Grant, but it doesn't prove conclusively that he actually is.