Welcome to the forum - thanks for coming here. I tend to find that folks here are quite realistic about what it is and what it isn't.
In my opinion, it may be that he wants to be poly - he is certainly saying the right things to indicate that he wants to be poly. However, what he is doing and has done isn't poly, it's cheating on your and your relationship.
If you were totally ok with this, and went along with it, then it is possible for this to turn into a poly relationship, but there has to be a lot of mending that goes on because of all the betrayal and lying. That doesn't mean that it's not possible, it just takes a ton of work.
The bottom line - as I understand your relationship, you both entered into this on a monogamous basis, and probably made promises and marriages vows along those lines (you know, the "forsaking all others" part). He, single-handedly, decided to change that, and expected you to just go along with it. When you didn't, he went ahead and lied and cheated to get what he wanted anyway. That is hurtful and disrespectful.
In order for you to stand a chance to make this work, the cheating and lying and concealment has to stop. He needs to regain your trust and the solidity of your relationship. Without that, I can't see anything being possible between the two of you, poly or not.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb