I chose to approach my honey, T-Rex, via email because all our face to face meetings were with a large group of friends (our gaming group) and we had no opportunities for private communication otherwise.
It worked out great. We've been together for about eight months now.
Mind you, my husband, Thumper, paved the way by mentioning that we'd decided to open our marriage in front of the entire group, on the pretext of asking our friends for advice about a girl he was interested in. T-Rex then went on vacation out of the country for a week, so he had some time to muse on it.
So, my situation is a bit different because there was some warm-up there. But I do think email is a fine way to do it.
One approach you could use would be to prime the pump, giving him a chance to absorb and process this new information (your poly nature) before taking the next step.
For example, you could tell him that you've been enjoying the playful flirtation between you two and just wanted him to know, just in case he was concerned, that this isn't going to cause any troubles as you are polyamorous, meaning open to having more than one relationship, and that your husband knows and is cool with this.
Then see how he responds to this information. If he doesn't respond in a clear or timely manner, then you might decide to spell things out more clearly.
I have found that the two sweet, wonderful, and intelligent men in my life are really
bad at picking up on hints (particularly from women) and that I must be super blunt and obvious about, well, just about everything... sometimes to an extent that seems absurd to me. Still, I think when introducing a concept like polyamory, there may be some benefit to giving the notion a bit of time to sink in.
Polyamory is foreign to most people, and it's a lot to take in all at once.
Then again, you could just go for it. I'm also a big fan of just going for it.