Hey. I can't contribute TOO much to this topic because I'm fairly early in my first poly relationship as well (a few months, really, most of which has been long distance). While I was visiting, this did come up a bit, though. We're a quad of one man and three women, one of whom has been married to A for over ten years. They married young, and in some ways the wife sees the circumstances of their marriage as detracting from their love, that if A found someone he was more compatible with, he'd leave her. While that's scary on an emotional level, it's scary on a practical level too, because they have four children, so the idea of being launched into single motherhood is pretty terrifying.
We've handled those fears thus far by reiterating that even IF that were a strong, realistic possibility, she wouldn't be single for long, because she's quite a catch. Besides that, after this long, I think if he really wanted out, he'd get out. I do think that it's just a matter of women (all humans, actually) in general being prone to insecurity and being able to manipulate ANY information into something personally hurtful to ourselves.
The only way to REALLY know is to let this play out, talk through everyone's feelings often, try to meet each other's needs for freedom/security/reassurance/stability along the way, and learn that he chooses to be with you every day. I would also think of that act that makes you say "if only he did ____, I'd know that he really does love me." He shouldn't have to prove himself constantly or whatnot, but once in a while, you do need that proof, that reassurance. Once you know what it is that would make you feel confident about his love for you, verbalize it to him, don't just expect him to be telepathic about it.