Well, this sounds similar to how my hubby and I got started down the poly path, and although the relationship with my other didn't work out I think the same may apply.
When I first started talking to Elric (the other) again and we started being flirty, I simply told Cajun (hubby) what was going on. I asked him if he was ok with this, and he was fine. When things with Elric started getting more personal and emotional, what did I do? I talked to Cajun again. I told him what was going on and again asked if this was ok. Things progressed via IM to the point where I thought Elric may be interested in a more physical relationship and not just cyber. This time I talked to Cajun before saying anything to Elric. This is also before we had a word for what might be happening, we only had our sci-fi readings to go back on. It took Cajun some time to figure out how he might feel, and how this could effect our current relationship, but in time he was willing to go slowly.
Although things fell apart romantically with Elric, Cajun and I have come a long way in the poly thought/lifestyle just from this one experience. Just take it slow and talk to him. Find ways that he might understand what poly is/can be and what it is not. One of the first and probably hardest realizations that Cajun came to was "Love is not a zero-sum game", ie. if you are giving it to someone else, he isn't getting the same amount. Once he absorbed that fact, he was willing to keep going.
Talk to you DH, there is no rush, you are not cheating...yet. Flirting is one thing, starting to cyber and discuss future plans with the Other can be construed as cheating. Just don't push him off the deep end by talking about one home living situations with shared time etc. I understand your DH may not be good at the emotional conversation, but if you having a poly life is to work, he has to be able to listen as well as discuss to a certain level the emotional impact of what is happening.
And just so you know, I never thought I'd be here either.
But this place and these people have been a tremendous help and a wellspring of knowledge for us. I wish you lots of luck!