Firstly- I am so relieved that this went off without any physical violence, and that the cafe owner protected you from too much verbal violence.
It is obvious that your exH is the one causing the problem here. The fact that you managed to calm things down and speak reasonably to your son tells us that. It was only when his father realised that you were having a conversation, rather than a shouting match, that he interfered and things kicked off.
Your ex is loading the gun for your son to fire - and I think he is making sure that your son stays stoked up and angry. I have a feeling that he fears your son's affection for you. It looks as though he constantly works to make sure that the two of you find no common ground. Your son is probably very conflicted. He has strong loyalties to his father, but I suspect that he would like to be close to you , too, but realises that his dad will cut him out of his life altogether (as he has done with your there children) if he shows any warmth towards you at all.
I was angry with your boy when I first read your posts, but now I am feeling very sorry for him. I think he may have a strong sense of being responsible for his dad - perhaps he didn't like the thought of his father being all on his own when you split, and felt he had to go with him to look after him. (He was only about 10/11 wasn't he? Children can have funny ideas and be very protective of their parents.)
Maybe he longed to be with you and the others, but didn't want to see his dad on his own. Poor boy, he must be terribly confused and conflicted, and now his waste-of-space dad is rattling his cage again, no doubt telling him that your replacing him, don't give a damn about him etc etc etc.
All you can do is make sure your son knows your door is always open to him should he decide to get in touch, and keep him n the loop regarding the new baby when s/he is born - weight, sex, name, etc.
I can understand now why you are so concerned about him, as well as loving him, but the important thing now is to look after yourself, let your DH look after you, and both of you care for this precious baby and your other children.
As for abortion - what a dick your ex is!