Agree with macdoodle
You say you know him pretty well, but also like you say, you don't know what goes on behind closed doors.
My situation - H and I were selling our home to move to a bigger home by a good school for ds.
He told his OW - we were selling our house, separating, and divorcing. I knew nothing of it all until we sold the house and he announced he actually wasn't coming to the new house with me!
Seriously that is the honest truth.
It took 6 months for me and the OW to actually realise we were both being played and to get to the bottom of the lies.
If it is meant to be for you both then it will happen. But please please - have some respect for the end of his marriage. Let him leave his wife and sort out his divorce. Let him sort his own head out and deal with the inevitable hurt that will come with the end of his marriage.
Then think about starting up a relationship with him.
At the end of the day, at this point in time he is still married and you can never be 100% sure that you know the full truth.
For your own sake as well as his wife's, try to take a step back. You can be a friend without letting it go any further just now.
And I also think that would be the right thing to do for the sake of yours and his future relationship - it sounds as though you would definitely prefer not to be the 'OW'. So don't be. Be his friend until he has moved out and his marriage is 100% over. Then you and him can start a fresh with no baggage or accusations from his ex. I really believe that would be best for all of you.