The terms "primary" and "secondary" in poly mean different things to different people - there have been long discussions on this board about that very subject.
I think that rather than concerning yourself with labelling what it is or what it can be, you instead spend some time thinking about what it is you want. You said in your earlier post wanting to spend "as much time as possible" with him - what does that mean to you?
What trappings of a relationship are important to you? You said that you weren't planning on moving in with him (which is what, by the way, some people think of as what "primary" means, just to give you an idea)... so what would you want or need this relationship to be like?
How much communication have you had with his current partner? My personal way of doing things like this is to get everyone together, whether it's in a room, or on Skype of FaceTime or whatever, and actually get everyone to talk about what their own needs are. See if there is enough common ground there to make it work.
"Listen, or your tongue will make you deaf." - Native American Proverb