What you want for your son and his dad is unfortunately irrelevant. If you want to parent your ex for the rest of your life, then you can try to have a say or influence.
It is the hardest thing out there to do - what your ex wants for his son and him is what is going to drive this issue.
Stay strong. People always ask how I do it. I simply tell them that nobody gave me the option not to when I got up this morning. And then forgive yourself because you will parent differently now that you are solo. Different does not mean worse or better. Just means different. So, when he is cranky - make him breakfast for dinner. Or have Burger King in bed. And then forgive yourself - he will be happier and you will move on to the next day!