Bearing in mind that John said his father was Henry Grigg, nevertheless I thought I would have a look at the 12 John baptisms that KGarrard mentioned, plus one nonconformist birth for the timeframe.
I can rule out seven because they were either buried before 1859, or they were in Cornwall in 1861, when my John was in India.
Year Parish Parents Outcome
1835 Boyton John & Martha buried 1836 Boyton, infant
1835 Gwithian William & Mary Ann buried 1848 Perranzabuloe, aged 13
1835 St Dennis Richard & Mary 41,51,61 with parents in St Dennis, 1867 married Mary Ann Allen buried 1912
1836 Roche Joseph & Mary Ann buried 1855 St Minver, aged 19
1836 Kilkhampton John & Susanna buried 1855 Kilkhampton, aged 18
1843 Crowan John & Sally 51, 61 with parents in Camborne
1845 Warbstow Richard & Mary Ann 51, 61, 71 with parents in Warbstow
Can anyone help with the other six?
1835 St Dennis Nicholas & Elizabeth CANT FIND AFTER BAPT parents in St Dennis 41-71 another son John born c1852
1835 St Ervan Joseph & Philippa 41, 51 with widowed mother at St Dennis NO SIGN
Bible Christian [of St Dennis, born St Columb] AFTER
1837 Kilkhampton Charles & Eliza 41, 51 with parents at Lewdon NOT SIGN AFTER
1838 Lanlivery John & Mary Ann 41 with parents at Redmoor NO SIGN AFTER
1841 Jacobstow Humphry & Mary 51 with parents at Jacobstow NO SIGN AFTER
1842 St Michael Caerhays John & Margaret CANT FIND AFTER BAPT parents at Altarnun 51 Calstock 61
Sorry about the formatting - I'm still coming to grips with posting!
Looking at the census there are other possible John Griggs not listed above. I will try to get my head around those.