Hi Sunnylyn
I'm guessing you have the listing for Ivan from the familysearch website, where it also shows parents as : Thomas William FIVEASH and Jessie BOYER ?
[As it's a "submitted" record and no source has been quoted, it should of course, be treated with caution. ]
The NZ BDM death index gives Ivan's age at death in 1958, as 42 years (born about 1916) - but that could well be incorrect.
I'm assuming that it is Jessie Thomson FIVEASH born about 1868 who is thought to have been the mother of Ivan ? If so, then she would have been about 46 / 47 years old at the time of his birth ? [That age though doesn't necessarily rule her out of being his birth mother. ]
Somebody may have access to a school record which gives a parent's name (one which actually states "father" or "mother", rather than "guardian" of the child) ?

~ Lu