You can take his phone away as a punishment, for a start. If he wants to contact his dad of course he can borrow your phone/your OH's phone/your landline if you have one (best option - might almost be worth getting one for that reason alone!).
You note down length of call, and 'charge' it to his phone when the punishment is over and he gets it back.
Your parents can of course keep in touch with him by contacting him when he's with you. Are they local?
I can't believe he has you all so terrified of him that you have to hide the baby clothes. HE'S TWELVE YEARS OLD. He's not your boss.
You have to draw a line under this and change your future - and his. Stop indulging him. Can you see that your own behaviour is irrational with regards to your eldest?
Break it all down step by step. Talk it through in minute detail with your OH, why you allow your child to rule you, how he has got himself into that position - or you have allowed him to step into it - and work out, step by painful step, how to stop it.
You need to be his parent. That means you make the rules and he follows them.
I know this isn't going to be easy for you, and your baby is due any day, but if you don't do this now, you're going to be too tired to do it when your baby is born and you are sleep deprived and exhausted. If you think it's hard now, it'll be worse then.
Your don't want your dd growing up with a repeat of your ex in the house treating her how your ex treated you; he will and you know he will.
So you have to toughen up now and strongly. You have to fight, you have to be forceful and sure. Your whole family's future (together and individually) is perched precariously on the top of this shit-pile, just waiting to sink into it. Build a bloody strong pillar to secure it, and do it now. You have a few days. Get on with it, it's nearly too late and you'll kick yourself forever if you don't.
This is going to require you to be superhuman.. If you get on with your parents and they're good people then discuss it with them, ask for their help to plan and keep you on the path. Keep your OH involved too. Involve anyone who could help you tease out a plan in record time and then help you carry it out and keep you at it.