I am aware that genealogy records are limited in South Africa, I believe census records are destroyed after statistics are taken and not much information is provided on certificates but i wonder if you or your mother may have seen any details of the death certificate.

Jack is usually considered to be an alternative to John so with that in mind:-

1901 census Isle of Man at 40 Senna Road Douglas.

William Crosbie 38 Hotel Porter, England (Liverpool)
Catherine 35 Wife England (Liverpool)
John James 20 Joiner England
William H. 10 Douglas
Catherine 5 England
Gladys M 3 Douglas
Joseph E 2 Douglas

Since William and Catherine were married in Douglas 1889 John may be from a previous marriage.

John James is the only John Crosbie i can't find in records but your Jack (john) may be after 1901. Was he a Joiner/Carpenter?

Len Faragher