So let me get this straight. You split with your boyfriend and jump into bed with a guy that you work with (bad idea number 1) and who has a fiancee (bad idea number 2) and who has a child with said fiancee (bad idea number 3.) And you seriously need help deciding what to do? Really?
He has said if it wasn't for his son he would leave
And of course you believe him. If he did not have a son, then he would tell you that he would leave her if not for the house they share, or the dog they have, or that she has some physical ailment that he needs to be there for her. There would be another reason. I know this because we hear these reasons all the time here from women just like you who are desperately looking for affirmation from others in order to continue to do something they know is wrong.
He also said he wants to know that the door will never be fully closed
In other words, he doesn't much care for your happiness as long as he can get in your pants whenever the mood strikes him. Again, why do you need us to tell you what to do?
He says he no longer loves his fiance
If I had a dollar for every woman who has written this statement about the married/attached man they are sleeping with, I would be rich. This line of bull is spoken by every cheating man or woman to the person they are cheating with in order to make themselves seem somehow less disgusting for stepping out on their spouse.
Look, Jespah, myself and probably EVERYONE who comments on here will tell you to drop the guy unless he splits with his fiancee. I will say this for what is probably the hundredth time on A2K. If the bastard will cheat on his fiancee, what makes you think he won't cheat on you?
Grow up and realize that this guy is getting a nice piece of ass on the side and that is all you really are to him. If he loved you, he would leave his fiancee.