You reconcile and your son goes back to last place and this guy will make it harder on him. He'll eventually shove him out of his own home if you cave
I was going to say exactly this.
OP, he wants to “be a family”. What has he done to establish a bond with your son?
Your son, IIRC is nearly in high school. He WILL become moody, grumpy, defiant, self-centred as he becomes a teen. Teens and adult men in the same house clash. They get through when they have a strong relationship forged by years of fostering that connection and by strong boundaries. Pragmatically, that looks like dads/step-dads involving themselves in their son’s sports, hobbies, school activities over time and with a degree of selflessness.
what has your ex-DP done to be a positive figure in your son’s life? How did your ex DP respond when one of his children openly resented your son?
If you get back together your son has no sanctuary. He will be living with a man who wants him there less, and a child who resents him. You’ll be heading towards high-conflict teen years, your ex-DP will want you to side with him when he feels your son is “disrespectful” to him. And that’s not mentioning his own poorly parented ND children and their teen storm that awaits you.
You love him more than he does you and he knows it
This is the crux of it, this man has damaged your self-worth, but if you love your son more than your ex prioritise your son. Give him the physical and emotional space to allow him to grow to manhood in a peaceful environment.