As I don't have a lot of advice (I mean I have one parent whos a swinger, one who's an ex-swinger/chronic cheater, but it's as alien to me as my being poly seems to be to them).
Is it being specifically requested that you swing with him to up his odds of having sex, or he's just said it would be easier and so its your idea to do that? I think everybody knows its pretty imperative not to have sex you don't want to have - the long term ramifications of doing so to please others is probably not worth it one bit.
Is he open to hookups on craiglist? Dating with the goal of finding a gf who will swing with him instead of you? (Have heard of some people doing that so everybody is getting their needs met and nobody is having to do something they dont want to) Nevertheless, if you are open to seeing if swinging would appeal to you, you could agree to go to a swing party and not get involved with anybody, to just check it out, with it being agreed that you'll see how you feel about it after the fact. It's certainly possible to go and just watch or wander around, or to end up having sex with each other but not other people there. I'd just make sure to set clear boundaries about expectations so you dont get pressured into anything you don't want to do.
Happiness will never come to those who fail to appreciate what they already have.