Is it possible that you see him as more of a life partner than you had seen previous partners? That could "up the ante" in terms of what you stand to "lose" if things go badly. That could, in turn, cause insecurities that you haven't felt before because you never had so much on the line.
I would try to avoid thinking that it's "him" because that can inhibit the process of dealing with these reactions, which are your reactions and not his.
There's also the possibility that your body is acutely aware of something that your mind is not seeing. Obviously I don't know him or the situation at all, I'm just saying it's possible that everything's not on the up-and-up, and that your body is trying to tell you something.
Personally, I've learned to trust my gut. There are times where things just don't "feel right" and I can't put my finger on it. I used to ignore those feelings because I couldn't rationalize them, and then in hindsight I would learn what I was reacting to. Now, when I have those feelings, I just get the hell outta Dodge and worry about "why" later.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker