Read as much as you can handle to get some things sorted out in your mind.
First, think about whether you would want to have more than one loving relationship at a time in your life. Think about how you would feel about him having more than one person that he loves and spends time with.
There are many, many ways of "doing poly". Some are very healthy, others less-so. You will see many many examples of both of them on forums like this one.
A site that contains some very useful articles about poly is Franklin's site at http://www.xeromag.com/fvpoly.html
- I can heartily recommend it.
And as redsirenn suggests, get to know him better as well - this is really early on in your relationship, so go slowly - all the moreso because this is new for you. Talk to him about his polyness - has he had multi-love relationships before... what sort of configuration were they... how did his two (or more) partners get along with each other.
Try to get a feel from him how his
poly works and do the mental exercise of putting yourself in the shoes of one of his loves and see how it fits.
All this is a part of getting to know any poly person in a dating context, whether it's your first time or your tenth, believe me.
I wish you luck on your journey