On a game with time-travel (Trevor's actions impact what Simon's environment will be), one can consider this:
A decision made by Trevor to still be able to help his son in the future, may require for him to make a sacrifice. Perhaps, in order to best help his son, Trevor has to become a creature of the night, thus he becomes Alucard.
However, a game with time travel usually has dimensional repercussions; a choice not made creates an entirely different universe. Couple this with a magical relic that's a "Mirror" (which by itself carries a number of connotations in terms of the magic of mirrors, etc.) and it's possible that what Cox said is both true and false at once:
Trevor is not Alucard.
However, "Alucard" can be a 'reflection' of Trevor; a magical being created by the Mirror whose lifespan abilities may allow it to help Simon in better ways than his father could.
"Alucard" could also be Trevor after he makes a choice that makes him Alucard, but if the game has time-changing elements, perhaps Simon can do something with the mirror to prevent this fate from befalling his father (like in that movie "Frequency", where actions in the future, relayed back in time, alter the events of time and prevent a father's death from happening).
All of these are story possibilities, as is the possibility of Sypha helping out her husband.
I am guessing the saga will be having Trevor, Simon, Alucard, and Sypha as the protagonists.