I'm sorry your ex is being a dick about things OP. I have been there and learned the hard way. Also, your parents are clearly not helpful and you need to stand up to them when they brush off your concerns. My parents are similar in that they used to label me as picky and hard work when I broke up with my abusive ex. I now stand up to comments like that and dont let them shame me for standing up for myself.
Going forward, I think you have alot of things to work on for yourself. First of all, you need to learn all you can about abusive men because then you will realise that they never change and there is no point reaching out to an abusive ex for comfort. He is just as bad as this man who is harassing you now and you have to cut all emotional ties with him. Yes it's really sad that you cant have a friendship with your sons father but you have to accept the reality that he is just waiting to kick you when you are down. Abusive men love to throw child custody in your face as a form of control and will stoop as low as they can go to get to you. Only speak to him about your DS and NOTHING else. Trust me, I've been there, done that and it never ended well.
I think you really need to stop dating for a while until you have healed from your previous abuse. Once leaving an abusive relationship, it can lead to a downward spiral of attracting further abusive relationships because you are not in a good place to spot the red flags. You did well on this occasion but he was pretty blatant with it and I worry you may fall for someone who is much more covert and hides it better. I think you are in a reactional state where you are looking for someone to protect you and fill the void your ex left but you have to realise that dating another man is not the answer. Getting validation from men is never the answer.
I have been in exactly the same place and it led to me being in 3 consecutive abusive relationships and I decided enough is enough and that I had to change my outlook. I have stopped dating altogether and learned as much as I can about abusive relationships. I still get lonely sometimes but I feel a million times better and I'm starting to get the real me back.
Invest in yourself, in the people who love you (not necessarily parents but your DS and friends) and take the time you need to heal from your past. Only when you can honestly say to yourself that you love yourself enough to not need a man and that you will not tolerate anything short of an equal loving relationship will you be ready to date.
Abusive men can smell vulnerability and unfortunately if you have been abused previously and not healed from this then you remain vulnerable. There are so many resources out there which can help and as I've said before, youtube video about codependency have really helped me.
You need to make sure you are healthy and safe so that you are able to take care of your son as even though your ex is chatting shit about getting custody, you could very well be jeopardising your DS if you find yourself in another relationship like this. Hard to hear but that is ultimately what snapped me out of my previous unhealthy patterns as my DS's mean more to me that finding love again.