It doesn't sound like he wants to be poly to me. It sounds like if anything he wants to continue to have options sexually to boost his ego.
I would focus more on your relationship with him instead of worrying about what he might be hiding from you. If your relationship is solid, he'll tell you when he needs to tell you. In the meantime, you can support his personality (because this flirtatiousness sounds like it is just kind of part of him) by going out and flirting, having a threesome -assuming that's something you're actually interested in-, and all that.
You might consider swinging vs. poly if it is just a sexual desire and not an emotional/relationship one.
All obviously just my opinion.