Could it be that your son is wanting to be "closer" to you as you are pregnant again? I know that my SD (who lives with us FT) gets a bit weird whenever her mum is pregnant. She doesn't get clingy as such but has said that she doesn't like it when her mum is pregnant.
I've noticed that you're seeking a psych visit 2x a fortnight. I'm wondering why as that seems like it would be quite intensive from the psychs and paeds that I've spoken with. Is there more going on behavioural wise with your son which could mean he may struggle a bit more at his dads? Do you know how he is when he is with his dad? Could you suggest using the communication book again? Does your son have contact with you when at his dads?
My DH works (a lot) so I'm the main parent that SD has. I know it's not always ideal but my DH spends time with her every morning and every night and of course on weekends (except when she visits with her mum). If he were with her mum he would probably only see her the same times as he would still work. It's not always about $$ - some fathers actually do just want to see/be with their kids. Maybe your ex, in his misguided way, is saying "I want to see my son" when he demands that he 'suck it up'. I'm sure he doesn't feel great if his son is not wanting to visit (I imagine you'd be devastated if the situation were reversed).
In an ideal world everyone would get along great and we could be one big happy family but unfortunately it doesn't always work that way. I do hope that your son can work on a way to have an awesome relationship with both of his families.
My advice would be to not use psych as a way to stop 50/50 but as a way for your soon to have a safe space and to work on ways in which he can better manage his feelings when transitioning between households.
If all that doesn't work, then maybe try re-negotiating your parenting plan at mediation and having an advocate for your son. I would try to be as honest and open with his dad and see if he can be involved/on board with what's happening at psych. It's not an easy road!