Even if John Maxwell Brew wrote a will before leaving the IOM, it wouldn't be recorded anywhere until he died - so possibly in Canada?

This looks like his father's family in the 1851 census:

Lonan parish Dist.4 Entry 34 Address: Crot Ne How
John 56 Married Head Proprietor., Farmer b.Lonan
Margaret 60 Married Wife b.Lonan
(followed by)
Lonan parish Dist.4 Entry 36 Address:Clay Head
Ann 76 Widow Head Farmer 24 acres b.Lonan
Saul 46 Unmarried Son b.Lonan
Ann 34 Unmarried Daughter b.Lonan
Thomas 32 Unmarried Son Baker b.Lonan
James 30 Unmarried Son Tailor b.Lonan

"To the memory of William Brew of Clay Head in this parish, son of Samuel Brew, who departed this life the 8 May ----. Thomas Brew, brother of the above, who departed this life on the 4 January 1852, aged 37. In memory of John Brew of Broit-ny-Howe in this Parish, who died March 11, 1875, aged 50 years [wrong age]. Also Margaret, wife of the above, who died January 31, 1862, aged 80 years. Also Ann wife of the above, John [Sam] Brew, who died July 9, 1888, aged 83 years. ?Jn & Mgt md 1813, Lonan; so Jn was born 16?..."
[From Joyce Oates's M.I.transcripts & her notes.]

Samuel [aged 46 in the 1851 census] marr. Elizabeth CUBBON 16 Aug 1851 in Onchan, and appears to have deducted a few years from his age for the 1861 census in Onchan Village, where he is listed with his 33 year old wife Elizabeth and children, including John Maxwell aged 7.
[Samuel 48 Married Head Agric'l. Labourer b.Lonan].

Father Samuel married Ann CANNEL 26 Aug 1802 in Lonan [son Samuel b.1803].
I suggest you look up father Samuel's will: Samuel BREW, 1827, Lonan, Arch, no.2, film no. 0106245

Sue