Tried it again, except this time I had more than one son.
Republic disbanded and he became feudal.
Tried again, and he was passed over in favour of second son.
Seems it works sometimes, sometimes not. I think the best case scenario is where you have only one son with primogeniture which leads to him being forced to being your heir.
When it worked, was your son the last member of your dynasty?
As far as I remember, that was one way of forcing the switch.
Obviously that's not a particularly useful strategy once you have a few hundred dynasty members. (Unless you go on a kin slaying binge.)