Hi Shiv,
Mr Mom gave very good advice. You will need to be the consistent and stable parent for your son.
I have a 2 year-old daughter who unfortunately gets invalidation, criticism, and other forms of what feel like emotional abuse from my uBPDw (a link to abbreviations is towards the very top of the page). Applying the lessons on these boards (links on the right of the page) have helped reduce conflict with my wife, which in turn has had some positive effect towards our daughter.
There have been lessons that are really helping with my daughter too. One in particular right now is validation, helping her to learn that I listen to her and care what she thinks and feels, which helps boost her self-esteem. Here is a link to a thread where parents of BPD children shared a really nice list of resources that may help children of BPD parents to grow up healthier than they may otherwise be:
https://bpdfamily.com/message_board/index.php?topic=202679.0My T (therapist) told me that as long as at least one parent is consistent, loving, and stable, then the child is much more likely not to develop a personality disorder themselves. The child is likely to benefit from therapy to work through other issues, but they are more likely not to suffer from a personality disorder.
Then there is the issue of what to do with your son in the moment, during a rage or harsh criticism. Many of us here walk the fine line between protecting our children and not wanting to undermine our spouse's parenting. For me, I suppose it depends on the severity of what my wife is doing.
There are some times when I try to find ways to take my daughter out of the situation, and days when I take her out of the house altogether to do something fun for her. At times when I validate my wife's feelings (e.g., frustration) but do not validate the invalid (e.g., ignore the actual complaint, especially if it is something like a complaint of my daughter acting like the 2 year-old that she is!), I find that my wife lets go of it more quickly and changes topics or goes off to do something else.
Unfortunately, there are still the times when my wife splits our daughter black (as they say) and either gets really angry at her or completely ignores her. This is when I remind my daughter that mommy's anger is not her fault. If she did something she should not have, I will still address it, but I help her learn that she is not responsible for her mom's extreme emotions and actions.
There are a number of members here who are going through or have gone through situations similar to yours. Please feel free to keep posting, and let us know how it goes.
zaqsert