I'm another mono and I feel for you. I wouldn't however go quite as far as Lemon Drop and say that this is just a way he can cheat and stay married. I don't think that is a call you can make until things have progressed further and you can judge just how much effort and compromise is forthcoming.
What you're dealing with is the infamous NRE and a conversation about this with your partner would be useful.
I do think the 9 days in November could be a real point of negotiation. I have been in our poly relationship for two years three months and I couldn't cope with 9 days. As much as I love my partner that would probably be a deal breaker for me.
Have a scout around my blog, there is a lot on there about my process of coming to terms with a polyamorous partner. While it is still probably too early, if you want to be happy in your relationship you are going to have to find some positives for yourself in having a polyamorous partner.