@Notsodomesticgoddess1 I wouldn't wish my situation on anyone. Every child deserves their parents. But something I had to learn the hard way was you cannot force someone to do something they don't want to. So when I stopped begging my sons dad he just stopped. It was harder for me than my son as I've never fully gotton over the rejection of my child, but life is much better now.
Your ExP sounds like a fabulous role model to your child. And I think its wonderful the part he played and his willingness to do it. But please don't think I am being nasty, I'm not, but he is not your sons Dad. I say this because clearly your problem here is the inconsistency of the man who is your sons dad and your desire to give your son a father figure. I get it. Its heartbreaking.
There is no reason your ExP can't remain in contact in some form. I imagine he will miss your child too, very much so, however you do need to make the break from physical contact for a while. In that time you need to start referring to him as Mommy's friend.
I've always been very open with my son. When he started asking questions it broke me. He is still so emotionally unattached which is great but he can never understand why I sometimes get upset... Hence why I said I've never gotton over the rejection of my child. If he asks me an awkward question, I won't answer him negatively, by slinging mud. I will tell him in terms a 4 year old will be able to process, take ownership of my part in things but still make the focus of our conversion about love.
As I say your problem is the biological father. Remember your child does not need anyone who doesn't need him. You need to make the point to his father how this behaviour and inconsistency will be damaging to your son long term which you won't tolerate. I know its hard but if he doesnt show or contact, focus your love and attention fully on your baby. Your baby will feel his mom's love more than the void of his dad at this age. I am not saying stop contact at all but stop trying to make it happen. He is a grown man who needs to own his responsibilities. Please just surround yourself and son with love for each other and have an open enough relationship for when the questions do start to flow. Though you have a while yet.
Some of the things my son has asked has floored me. My heart just drops. I can fight the tears now, but I will just answer him and then maybe ask him why. He just thinks it would be nice having a dad, but says he is really happy he has me, lol. I also tell him though that you cannot force anyone to do something they don't want to, how we have to be accountable for our own actions, how we don't force ourselves on anyone and if someone cannot see how unbelievabley amazing he is then it is their loss. The biggest thing is though I tell him I can't answer for someone else as I don't know why they do the things they do.
I have told you this because ultimately the dad is your problem. You have done nothing harmful to your baby. You are doing your absolute best. Please remember that xx