D.P.
Hi K.,
I'm sorry you are going through this. I know it will be sad for your son but he WILL get over this. You are his mom and he is your priority. That is evident from your post. If your boyfriend was "really close" to your son, he would never put out that type of unreasonable demand. I say you're better off without him. No man should come between anyone and their child (mother, father, grandparents, whatever.) Your son's father may not be the best dad--but he IS his dad, and clearly your son wants him in his life.
Please consider this scenario. I took a lot of flack in another post for suggesting that another mom stay single until her child was grown. I still stand by that advice. Let me CLARIFY: I'm not saying you shouldn't date or you should be celibate, I'm simply suggesting keeping dating life separate from family life. Too late for this particular "dad" scenario but this is exactly the kind of situation you can prevent from ever, ever, happening again IF you will consider, from this day forward, keeping your dating life separate from your family life. You can have overnights at your house IF your son is at a sitter's house, you can stay at a man's place IF your son is at your house with a trusted sitter, etc. No need for any of the men you will date in the future to meet your son until YOU know that the relationship merits the introduction. I would suggest never letting your son call ANYONE "Dad" again. Once introduced, your boyfriend can be "Bill" or "Tom" until they become his step dad, then he can be called "dad."
As for your current situation, again, I believe that someone who would give you that type of ultimatum has substandard character. If I were you, I would explain to your son that he has done nothing wrong, but you and "Dave" have decided to go your own ways for grown-up reasons, of which he will understand someday when he is older. Much like a divorce, you need to explain that "Dave" loves him and that "Dave and Mommy" just don't want to be together anymore, and make sure he understand that it was YOUR adult decision to end this relationship, not his fault.
I wish you the best of luck!