Hi Liz,
To add to what Averil wrote about looking up the JOHN CAIN Christening: 30 MAY 1859 Braddan, Mother: JANE CAIN, could you also please see whether there was a Presentment for his father, which should give his name and address, as well as Jane's name (the child was never named). They can be several years after the birth, when the father is brought to account to pay support, but would usually be within a year or two.
While you are looking through the documents to find Jane you could also watch out for Robert to see whether he has been brought to account for any other children. If you find Jane but with a different father for her son, you might still find Robert any time from 1859 onwards being made responsible for another woman's child who we could then follow up to see whether he was John.
I'm pretty sure they still had Presentments in the 1850s and Frances will let us know if I am wrong.
Sue