Hi RRE,
Welcome and hello

You are not alone, there are many here who have grandchildren with a BPD parent involved. It's pretty tough, for many of the reasons you mention.
Do you think your son is jealous of the relationship you have with your grandson?
There are some skills that can prevent things from getting worse, although they won't cure BPD/NPD. I like the saying, Do you want to be right, or do you want to be effective? It helped me look at things that might be effective, trying them out and seeing what worked and what didn't.
For BPD sufferers who have strong narcissistic traits, you may have to praise even the smallest things. With the person in my life who was very narcissistic (my son's N/BPD father), I found I had to acknowledge how he parented. "I heard so-and-so say you are so good at coaching son!" Or, if he walked in the door, I would say to him, "You're here! Let me put this down so I can focus on you and hear how you're doing."
People who are narcissistic tend to need lots of emotional validation in the form of praise.
I also had to give my son's father a feeling of control and importance. "You have a very busy schedule and it must be hard with so many people competing for your attention. Whatever works for your schedule is ok by me."
Stabilizing the relationship with your grandson is what makes it worth it.