I know this is easier said than done Ladysu but you're going to have to lower your expectations if you want to maintain your relationship with your son and d.i.l. and be a part of your GS's life when he's born.
There's no point in looking back to how you helped your mum and dad when she was ill, and being angry or disappointed or frustrated, if your son sees his role and responsibilities differently.
I think you should take some time to look at what makes your relationship with him easier, and what doesn't. What you've tried in the past that works, and what doesn't.
As frustrating as it is, keeping any conversations light and simple, and avoiding his triggers which include living with them and if and when you'll see your GS, works. You're both able to remain calm and his visits are more pleasurable.
I suggested the other day that if you're talking about when you move out of your dad's and where you're thinking of moving too, make it a positive conversation. Tell him about any plans you may have and the arrangements to get the job done without asking him too, or making him feel that he should be helping in some way.
What I'm trying to say is let him see you as someone who isn't totally reliable on him for your happiness and all of your emotional and practical support.
Bombarding him with text messages doesn't work. As far as I can tell, he 'phones you every Saturday so wait for his call, and was talking to his m.i.l. yesterday really a good idea? Are they more than likely going to find out you did this, and see this as you checking up on what they are or are not doing?
TBH, even without everything else that's going on, I'd have been very annoyed if my m.i.l. had contacted my mum on Mothers day, and Mr. S. would have been too.
It does seem like I have to fall into line whilst he dictates what's happening here, yes it does look that way. Is it right? Well relationships should be reciprocal and healthy ones usually are but if this is how your relationship is going to be moving forward, only you can decide if this is the relationship you want and can live with.
I don't think that reluctantly accepting it is going to work in the long term if things remain as they are, because you need to understand that unless things change, and only he can make changes, this is how it will be.
I do not mean that you should ever accept his verbal abuse, that is completely unacceptable regardless of how stressed he may be feeling.
With just a few weeks until the baby is due, if he 'phones on Saturday and says he's calling on Sunday, regardless of how much you want to see him, please think about telling him you'd love too but if he thinks he should focus on his wife and getting ready for the new arrival, you'll be happy with a chat on the 'phone for the time being.
Give him space, he'll thank you for it.