Right.
First things first.
(((HUG)))
You love are a fabulously brave and very intelligent woman. We can see that, cos you have worked this out, and you are now here asking us these questions.
To answer the questions in your title: 1: NO You can not SAVE 'us' and 2: He's not being an idiot, he's being abusive, manipulative, controlling and menacing. he is also abusing your son directly. An idiot would not be capable of any of the above. He is doing ALL of this in FULL knowledge of his actions and the effects they have on you. Sadly, your DS doesn't figure at all in his calculations, he doesn't care about him, your DS, to him is merely collateral damage. ANYTHING is justified in his pursuit to get at YOU. He HATES YOU.
WRT getting your DS and yourself away from this evil man, absolutely you have to do this as soon as you possibly can. Come hell or high water, with or without your Mother's support. There is no Plan B to that.
Believe me, the LAST thing you need to think about is exposing your DS to his father. It will do him NO good at all. FWIW, the ideal scenario would be for your DS to never clap eyes on his father again. This guy has ruined and poisoned every aspect of your life and your family unit. HE HAS.
If you remember nothing else, Remember this: YOU are not ripping the relationship to pieces. YOU are not denying this man a life with you and his son. YOU are not making it impossible to stay with him. In short YOU have NOTHING to do with this at all.
Please read Lundy Bancroft Why Does He Do That.
Please call Woman's Aid, or Respect if you can't get through (they are govt funded, set up for perpetrators, but perps don't actually seek help too often, so are more likely to be twiddling thumbs)
You will NEVER be able to fix this. You didn't break it. HE DID.
IT WILL NEVER GET ANY BETTER, ONLY WORSE. He will escalate it, because they ALWAYS DO.
Get help, get out.
Let us know how we can best help you to do this?