Sorry - further development! I've just found John Cain on the 1841 census. (I don't know how I missed him in the past, as I've been through the records so many times - perhaps it's because his age doesn't quite match the other records.)

He is living at Ballamoday (Ballamodda), E Baldwin. He is an apprentice blacksmith, aged 15. The head of the household is Matthias Cain, aged 75, a schoolmaster.