Generally a main requirement for family reunification is that you would be able to provide financial support for yourself, your son in Germany and your son who you wish to bring here. As you are not working and in receipt of Jobcenter support, this requirement is not fulfilled and family reunification is not possible. You can of course still apply and detail the hardship circumstances but these details will be less relevant to the authorities than your financial situation.
You can see from the information about this type of visa on their website that this is a requirement:
If you could manage to just have a part-time job, your chances would be better. The job income plus Kindergeld for two children plus potential child support payments may be enough to show them that you can cover your living costs.
It is also not necessary to travel to the consulate for a visa application. You can provide someone in South Africa (e.g. a family member) with a power of attorney to accompany your son to the consulate and through the application process.