The Deceased Estates sometimes contain wills, and lots of family information (sometimes including parents), but there's no guarantee.
You usually have to get someone to get a copy of the record for you. There are a number of researchers that will do that for a small fee, if you can't go yourself, or if the archive won't send a copy. The EGGSA also offers a copy service for some archives.http://www.national.archives.gov.za/NARS%20DMLIB%208372%20Ma...http://www.eggsa.org/sales/eshop_e_dc_docs.htm
Here's a few potential candidates. There are several with middle names, but you haven't mentioned a middle name.
Name: MCCALLUM, John
Source Location: Cape Town Archives Repository
Source: Master's Office / Orphan Chamber, Cape Town (MOOC)
Collection Name: Cape Estates Death Notice Index 1834-1951
DESCRIPTION: MCCALLUM, JOHN.
REMARKS SURVIVING SPOUSE ISOBELLA MILLER MCCALLUM. (BORN STUART).
DESCRIPTION:MCCALLUM, JOHN. BORN IN SCOTLAND. S SP MCCALLUM, LAURA ALICE. BORN HOLWELL. PRSP MCCALLUM, KATHERINE. BORN MCFARLANE. DECEASED ESTATE NO. 4561/1919. DECEASED ESTATE.
DESCRIPTION: MCCALLUM, JOHN. ALSO KNOWN AS JOHN IAN MCCALLUM. BORN IN ADDIEWELL, WEST LOTHIAN, SCOTLAND. S/SP MARGARET TEMBY MCCALLUM, (BORN MCFARLANE). DECEASED ESTATE.