Why is Robert Kardashian hiding is mysterious son?

The Kardashians are famous for their open book life due to the reality TV series, Keeping up with the Kardashians. However, it seems Robert does not want people to know about his son whom he tweeted about recently. Robert is also battling with drugs and weight issues which has led to depression but he seems to be recuperating from his health issues. The question is who is the  anonymous mother of his son?

See his recent pics with sisters below...