I might guess wrong here. And I'm not sure what their current agreements are... but it sounds like he was willing to have an emotional affair with you, and came close to a physical affair, then felt guilty. That doesn't sound like him dating you on the up and up. It sounds like cheating on some level. (Why are you up for cheating on any level? Do you not think you deserve to be dated on the up and up?)
I agree with TenK -- don't pin you hopes here. If you are thinking polyamory can help you still keep going with him while he's thinking of ending it... I think you are both better off letting a cheating affair end.
Even if he comes clean to his wife, she might not be up for poly at all. Or even if she is up for it, she might not be up for poly with YOU in the network -- his former cheating partner. There's trust betrayals there. And part of the price of admission to opening their marriage might be ditching any cheating partners. (you) The cheating start may have blown it for the both of you.
http://felislunae.org/relationships-love/coming-clean/
It isn't like poly is cheat proof either. He's currently cheating on his wife to some degree -- be it emotional or physical. If you were in a polyship with him, how could you trust him not to cheat on his poly agreements with
you? Based on what behavior?
If you want to poly -- go ahead and poly date. Bring it up from the beginning as you date and weed out the duds. Maybe something like...
"I like you. I want to date you. I also want you to know that I am not monogamous and I'm not looking for an exclusive relationship. I am seeking polyamory."
- How do I bring up the subject of Polyamory to him?
Could keep it simple.
- I don't want him to think I am "freaky"
You cannot control what he thinks. He controls that. So I grey that out.
- (I want him to know I am) open to the option of being with him and her exclusively.
Why would you want to be exclusive to them so fast? Rather than get to know them before making any
more promises or agreements?

Do not agree, promise or offer stuff you cannot follow through on. Better to wait and make agreements more slowly so they can actually be kept.
Could simply let him know what you are and are not up for at this time. Could say something like...
"I am no longer up for cheating stuff. I am open to dating you in an open, honest poly V with you as the shared sweetie if you and your wife are also up for that. Could you be willing to let me know where you stand?"
Then sit back and let him digest that and wait for his response.
In your shoes? I wouldn't put much more into things with him -- if your interest is poly? It seems easier to poly date someone else who doesn't come with all this mess attached. But if you want to know for sure -- you have to ask. So spit it out. I just wouldn't pin my hopes on a cheating situation transforming into a healthy V -- that's pretty rare.
Galagirl