Well, you just find a time and sit down and talk about the differences between monogamy and non monogamy and tell him you have been recently thinking about whether non-monogamy is an option or not in your marriage.
You will never know how he feels about it if you don't address it with him. The fear itself is worse than just feeling him out about what he thinks and feels.
If you have done much reading here, and I encourage you to do so, then you will be able to prepare yourself for the many ways he might respond. Be prepared for any response and allow him the time to really think about it and get back to you. He may have never even considered it an option, after all.
Best of luck to you.