To be completely honest, the guy from your past sounds like a total schmuck by the way he discards partners and is expecting you to be with no one else in order to be with him, while he is with other people. How is that supposed to work, anyway, being that you're married? If I were you, I'd tell him to take a hike.
It sounds like you're looking for some way out and are willing to put up with what he's proposed out of desperation. Don't choose to be a liar and cheater. Does your husband really deserve that? Do you need that kind of hit to your self-esteem to be a secret mistress, hiding and telling lies? To be with someone who sounds like he would only use you anyway?
I think SNeacail said it very well. Get into counseling or therapy and work on your marriage. Figure out what you want and learn how to communicate with your husband. Stay or leave, but don't cheat.