Hi and welcome! First of all, do not presume or assume anything. If you want to know whether or not he considers his relationship to be "primary," then ask him. Many poly people do not apply hierarchies to their relationships, and see their partners all as equally important, so it shouldn't be an automatic assumption on your part.
If it is primary for him, that also does not mean that any other relationships he has will always be "secondary." There are many ways to live polyamorously. That being said, when someone does have a primary, obviously everyone they meet and want a relationship with will naturally have to be secondary or whatever. We cannot all be someone's primary, so it is quite common to start off in a relationship as something other than primary. Read this Secondary's Bill of Rights on this page: http://www.morethantwo.com/polyforsecondaries.html
You should feel free to ask him questions and find out how he conducts his relationships, what safer sex practices he and they use, and if there are any agreements he has with other people that will affect you. Just ask. As the relationship progresses, if you feel you want to continue, you may want some contact with his other lover, which can help to take away the fear of the unknown.
But it is new, and you are just getting to know him. Enjoy this budding relationship and don't get too hung up on where it's going yet! There is plenty of time for negotiations and maneuvering, you don't need to get into all that during the initial dating stages, except for just establishing honesty and good communication.
Secondly, don't make it a self-fulfilling prophesy that you will become full of jealousy about this. I always have a chuckle when people are starting out and say, "I'm afraid I will be too jealous" -- but until you are in the thick of a situation, you don't know what your response or reaction will be. YOu may find yourself more comfortable with it than you ever thought you would be.
Just treat it like any other monogamous relationship in the sense that you would ask for what you need, voice concerns, stand up for yourself when needed, and be sure to define your own personal boundaries. But since he's poly, read up as much as you can about it -- there are lots of good info in our Golden Nuggets section of the forum.
All the best!