He must have another (second) name as well. As Johannes is one of the main family names for Slabbert it is almost impossible to know what one it is. I have 97 different Johannes's on file with no less than 17 falling within a 10 year timeframe from your date so..... I can however not find the grandmother on file, so I presume this would be one without detail of the wife on my records! However should he have been born before 1930, I should have him on file!
Should you be able to supply a bit more detail I am sure we would be able to help with this as well as the full register back to around 1600. I would then also appreciate the detail of your Grandmother to update the tree as well.
Regards Lional