He is a “half demon” or so he believes.
If he is Dis Pater, he’s also phoenixed.
What Phaestus does not say in this book is that Orcus’ son “just appeared” after Orcus Phoenix Cycled a previous time (probably the last time)
I could make that clearer by saying at that if it makes things more intriguing.
They actually are related, but it’s not as simple as Rupert believes. As we know from Reggie, the demon that fathered Rupert does not have to be the actual father. Not saying that the demon was an incubus, but it could have been.