Gilbert and family were still in Scotland in 1861 and are enumerated at Clauchan, Bute as Candlish.
Gilbert Candlish 36 , Shepherd born Kirkoswald Ayrshire
Elizabeth 36 born Girvan, Ayrshire
Alexander 12 born Straiton, Ayrshire
Jane 10 born Straiton
Margaret 4 born Straiton
Gilbert 2 born Straiton
As you can see there is no mention of John.
The same family is enumerated at Glenkil, South Lamlash, Bute ( as McCandlish) in 1851 with children John age 7 born Girvan, Elizabeth age 5 born Straiton, alexander age 2 born Girvan and Jane
age 4m born Girvan
Most of the births plus that of another Gilbert b 1854 died 1855 can be found on Familysearch.
also the marriage of Gilbert to Elizabeth Clarke in November 1843.
Familysearch also has a copy the passenger list showing Gilbert McCandlish, Shepherd and family arriving in New Zealand in Jan 1864 which includes children Elizabeth 18, Alexander 16, Jean 14,
John 10, Margaret 6 and Gilbert 4.
I looked for a possible John Telfer in 1861 and found the family of John Telpher , age 59, Farmer and wife Margaret ( whose maiden name appears to be Clarke) in Straiton with a son the correct age. Strangely they appear to have two sons called John, one age 8 and another age 1 (John James Telfer born 1860 in Straiton) John and Margaret married in Straiton in Nov 1843 so it is possible the first John was born to Margaret before she married. Margaret nee Clarke was from Girvan and I believe she may be a sister of Gilbert's wife Elizabeth and that your John born 1853 may therefore have been Gilbert and Elizabeth's nephew. If he wasn't John Telfer's son then that might be a reason why it seemed a good idea for him to accompany his aunt and uncle to New Zealand.
Isobel