If your husband wants to be polyamorous I would say that you have some serious discussions ahead of you.
Firstly, you need to discover the reasons why he is pushing for this. Is he curious to experiment, or is it a reflection of being unhappy in your current relationship? Only the two of you can figure this out.
You also need to work out how you feel about the proposition. There are plenty of people in polyamorous relationships who are happy, and making it work for them, but both parties need to be completely committed to the idea, and clear rules need to be in place. If this is not something that you want then I would not advise going into it half-heartedly to try and keep your husband happy.