Yes indeed he should be allowed to say anything, and ofcourse also be punished for calling for a race genocide.
As
NarrowBandFtw posted above, this is a straight forward case: Inciting the murder of a group of people in the country based due to racial hatred is a slam dunk situation of an illegal action.
If, however, he posted "Frack White people" - then there's no threat of violence or killing, and it should therefore not be punished in a courtroom as it doesn't necessarily break any laws that I'm aware of.
That said, an employer can absolutely warn/fire him for such a statement as he's made his racism known in public which undermines his relationship with colleagues and places his company in disrepute.