Familysearch has a John Cain Christened 21 September 1823 Braddan, Isle of Man
Parents THOS Cain/Eleanor KELLY
Image has him as Illegitimate*
Maybe?
Parents MATTHIASCain and Isabella FAYLE chrsietened Marown;
THOS 23 Feb 1801
ISABEL 24 April 1803
MATTHIAS 9 Feb 1806
So guess this is the Will info?
Describe The Record (Notes)
Matthias Cain wd 4 July 1851, adm 1852, Marown, schoolmaster Baldwin; dau Isabella Cowley als Cain wife of James Cowley miller both beyond the seas; gdau Isabell Cain dau of decd*** son Thomas; son Matthias exor.
Thos Cain buried Braddan 12 October 1839 age 36
That's fabulous, thank you! While it's not 100% conclusive, I think you might have cracked it! I'd already rather tentatively linked William (John's son) with the William Cain, aged 28, who appears on the 1891 census living with an uncle, William Kelly, at Tromode, Onchan. (After the murder-suicide, William lived first with Catherine's brother, Charles Shimmin, and then with another Kelly family at Renscault, Braddan.)
So that would all fit, if John were the illegitimate son of Thomas and Eleanor. It looks like Thomas's children moved in with their grandfather Matthias after their father's death.
Thanks a million!