I second what monaural said about timing! (But, as a pansexual poly, I want to just mention that 'taking on a girlfriend instead' is not as easy as it sounds. I can't choose who I fall for by gender, honestly. It's not a matter of picking out a pair of jeans, for me. You probably didn't mean it that way but I just felt the need to point that out.)
I've recently brought up the matter of my poly nature to my partner of 1.5 years (his username is Bold, on the forums, if you're curious) and we're working through things slowly together. I, like you, pushed it under the rug since we started dating but it kept bobbing to the surface again so I finally just put it out there again.
He needs a whole lot of extra love, attention and reassurance right now. Mono people think very differently from me, I have come to learn, so there may be a lot of assumptions that he might make about your motivation for wanting poly. If he's going to be away from you physically and out of easy contact via phone/internet/etc then it's not a good time to start a secondary relationship, in my humble opinion.
The stomach turn reaction is also something that my partner experiences so I can relate to that.
Welcome to the forum. :3