Simon IS Trevor's Son... in LoS-Verse.
But in that universe, Trevor is Gabriel's Son.
And Gabriel becomes Dracula.
And when Trevor goes up to Gabriel and eventually becomes turned, he foregoes his Trevor name and goes by Alucard
As for the more expanded Classic/IGA-verse, they're never called out on their exact lineage, except for a few:
-Leon Belmont is the first recorded Belmont in the IGA/Classic timeline.
-We know Trevor Raphael Christopher* Belmont married Sypha Fernandez/Belnades (mentioned in Harmony of Dissonance's developer notes/interviews, confirmed in the Castlevania Judgment dialogue)
-We know for a fact that Soleiyu is Christopher's son.
-We know Simon is descendant of Trevor. However, he is not Trevor's Son.
-We know Juste is Simon's Grandson
*fixed, after finding out Cornelius is only a fanon surname, and in the IGA conversations, he mentions the C being for Christopher, to tie him somewhat to his descendant, Christopher Belmont.
--from the Castlevania: Harmony of Dissonance script--
Maxim: Long ago, you told me of Dracula's remains. Do you remember that?
Juste: Yes, I remember. The story of how my grandfather, Simon, gathered Dracula's remains and destroyed them to undo his curse.
-We know Jonathan Morris is John Morris's Son, and John is Quincy Morris's Son.
-We know the last known Belmonts in the IGA timeline are Julius Belmont and his little sister, Kokoro Belmont.
These are the facts about the Belmont Family.
Now for the speculation. These is all conjecture, unfounded, only bits and pieces have been revealed, not enough to connect any dots meaningfully:
-Sonia is Descendant of Leon through her Grandfather
-Some rumors exist that Trevor is Sonia's Son (dates are very close together)
-Other rumors exist that Trevor is Sonia and Alucard's Son (some speculation based on the events in Legends, but very vague)
-Other rumors exist that Trevor is a brother or a cousin to Sonia, but not a direct descendant.
-Christopher Belmont is Trevor's Grandston (dates match up)
-Simon is Soleiyu's Son (dates match up)
-Richter is Juste's Grandston (dates match up)
a disappearance of Belmonts happens at this time, making it difficult to know much about the name "Belmont"
-Reinhardt Schneider is a Belmont descendant (the truth). It is unconfirmed how direct of a descendant from Richter Belmont, the last recorded Belmont so far, this descendant is (the speculation)
-Quincy Morris is a Belmont descendant (the truth). It is unconfirmed how direct of a descendant from Richter Belmont, the last recorded Belmont so far, this descendant is (the speculation).
-There are rumors that claim that the Schneiders and Morris clans are cousin clans to Belmonts, with a common Belmont ancestor (female Belmonts have existed in the past, and I don't think it would be uncommon for a few daughters to have appeared so far, besides Sonia)