I'd put a hold on all contact. I hope you don't mind, but I forwarded the thread to a friend who has experience of this, and she's provided some advice.
Firstly, report the abusive behaviour to the police. This would be the phone calls, and the threats to take your DS. That falls under emotional abuse, and I may be mistaken, but I think him using your PTSD against you falls under psychological abuse.
Secondly, it may be worth talking to social services. They're on your side, and please try not to worry. You will need to tell them everything, and they will help. This is a child protection issue, as your ex is acting in an abusive manner. Do not allow any unsupervised access at all; If you can withold all access, please do so.
Thirdly. Change solicitors, pronto and apply for a residency order. He can't just turn up at court and get custody... He's talking out of his arse cos he knows this scares you. You would need to be invited to any custody hearing,
Fourthly. Apply for WTC if you're not claiming. Also, contact the CSA and check that the amounts you are receiving are correct. Ask them to apply an attatchment of earnings, which means he won't be able to mess you about with regards to payments.
Hope this is some help to you.
Do you have support right now?