Not necessarily true. Mirror of Fate takes place 25 years after LoS, making Trevor 25 if he was born during LoS (which is the age he looks), or older if he was born before. On top of that, he was raised by the order, which means when Gabriel got someone pregnant, the order still had the means of hiding his kid. If Trevor was a vampire kid, he wouldn't be the son he never knew he had.
No, im sticking by my wacky theory just like everyone else is. Gabriel mated with a vampire then Trevor mated with a human so that he could create a dhampire with 1/8th of vampire blood. And in case he died or something the brotherhood could send their 1/8th dhampire (and apparent beastmaster) Simon Belmont to look for him. And then later he will find Trevor in the castle brainwashed and a vampire.
You know, im starting to like this "wacky theory" gimmick I got going on. It's kinda fun in fact.
Oh and I noticed nobody other than G4 has picked this up, you would think that it would be circulating around the web by now.