i've been here.
is having two children really worth your relationship?
as i understand your post, you never discussed how many you would have and therefore he isn't going back on a promise.
i think that the only thing to do here is to really examine in a deep way, what it is that makes you so desperate for another child? there is an inevitability that this will be as much about 'your stuff' as an altruistic wish to have a sibling 'for' your son.
it's really strange how the mind works. I was absolutely desperate as people here may remember. As soon as the gap between my sister and I had passed, the sense of desperate urgency went. I'm not saying that is or will be the case with you, but if you can find out what the underlying 'need' is, you may be able to diffuse this for yourself.
time changes everything. feeling trapped is not conducive to changing your mind. if you can let go of this a little (your son is only 14 months remember, things are going to change so much in the next year in terms of how much more enjoyable he will become) then maybe your dh will be able to let go too.
i'm not saying that overnight you'll both suddenly reach a point of ultimate understanding...but you'll find if you can reliquish your grip on this as the most important thing in the world, that maybe his position will become more flexible too.
the relationship predates the child. if you came together in love, did you do so only in order to have children. surely it is worth putting your relationship first.
i'm trying to tell you my experience. a year ago i was where you are (except dh most certainly was not in the guest room!!! ).
now, we are both in a better place and are both willing to be open to what happens ttc wise.
i've realised that my relationship is more important to me than anything. it is the basis on which my family is built. i've also realised that there was a lot going on for me in terms of trying to repeat the experience that i had (which is impossible). interestingly, i think that actually the experience of having a sibling arrive was rather painful for me (as for many firstborn). dh has realised that he has lots of stuff around siblings going on and that it has been very hard for him to adjust to parenthood in some ways (he is absolutely as hands on as i am...which cannot be said for most fathers).
i hope some of this helps you.