I would suggest he come here and read the NUMEROUS posts and threads on mono/poly dynamics that can be found in the tags within the "search" engine. There are some very knowledgeable mono people on this forum and he is not alone.
Mono and I, my boyfriend that I live with, have run the gamut of talk about this topic and we conduct workshops on this topic locally. We like to think of it as mono and poly are from two different cultures.
When one goes to a foreign country there is a sense of awe and wonderment. This is important to have when talking to someone that is mono when you are poly, or visa versa. Saying that you "don't believe it" is not an option. Just as it wouldn't be in a different country to say you don't believe that they are a different culture.
The other point that might help is that its my belief that some people like to live a poly lifestyle to spruce up their lives. This is much like dating or swinging, but with more love, open communication and honesty between those involved... some are genuinely born poly, like someone who is gay.... your partner is half right on that one...
Bottom line is to find what works, discuss options, gain some understanding about what is going on for each other and respect that as their truth.
There are other threads that might be helpful to you here in learning more about what poly is to others... I highly suggest reading around a bit, doing tag searches and getting more acquainted with other peoples reality in their dynamics so as to have a sense of what to expect.