NYCindie and Anneintherain bring up some great valid points that offer insight into the minds and struggles of people involved with a married/partnered person.
If you would like to take a look inside a man who is possibly similar in nature to your boyfriend, I'm going to suggest reading Redpepper's blog and some of the links off my profile. We've been doing this for 2.5 years and from what you are describing, I think there could be some strong congruences. The fact that you are invested seriously or attached, however you want to put it, tells me you value a retentivity with this partner. Things to talk about specifically:
Boundaries - what is he ok with, what do you want
Compromise - we don't usually get everything we want...what is a healthy balance for both of you.
Bottom Line - do you or him have a breaking point that you can identify
Here is a link to some specific mono/poly discussion:
Survival guide for dating a mono
If he is truly mono then he has got to ask himself some very big questions depending on his back ground and you could ask yourself some big ones too. Here is a link that might shed some light on things: The gender is reversed but it might still apply.
More than one person on here has experienced this situation. I can think of two (including myself) where a partner is not happy with an open approach to poly and the struggle ensues. We still struggle with this issue after more than two years. If you both really care about each other and want this to work, you are not only in for a potentially rough ride...but also a long one.
Also....More than one guy exhibits this non-jealousy trait towards a married womans' husband. I have in the past as have other friends of mine who were having affairs with married women. The sex with their husband doesn't even register for the guys I talked with about this (non of which even heard of the concept of poly, hence the affairs) but the sex with people or sometimes just men other than their husbands was a complete turnoff or trigger for jealousy. For me personally, the marriage status of Redpepper and Polynerdist over rides a lot of my own fundamental responses to the idea of "sharing" a women as a monogamous man (internally mono by nature...not socially conditioned mono)
At some point, it might just come down to cutting your losses and both of you moving on. Better to have a healthy short term relationship then a toxic long term one. You have to live the way you need to and so does he. Both are valid and both have to be elevated above the idea of "your shit" and "My shit". If you use words like that, even just in your head, you likely won't be able to overcome this. Sometimes, the root of this stuff isn't jealousy at all or any insecurity. Sometimes people are just different and not compatible.
This of course is all based on my own experiences and may not reflect the reality of any other human being. Soooooo..none of it could apply. He could just be very insecure and seeing how it has only been 9 months, that is likely.