I start to doubt myself when my replies feel a little less than 'natural/authentic' and am always concerned he will see through them.
I know exactly how you feel! I'm so glad you found us here! Before I found this site, I think DOUBT was becoming my middle name because my own common sense didn't work. In fact, NOTHING I was doing worked. Coming here to vent has been such a blessing! And the tools really help, though I discovered I can only validate well when I let go of any resentment or self-pity I've been feeling. When I need to be validated, I come here.
I did validate, I used SET and spent some considerable time explaining how GS will see things clearly for himself in time if my DS keeps his temper and behaves calmly. He struggles soo much with that and his ex knows exactly how to push every one of his buttons.
Your DS and GS are so lucky to have you in their corner! That's great that your DS heard you about staying calm. Maybe this situation will motivate him to learn some new emotional skills. I feel badly for them. This is such a difficult situation, even without BPD/NPD in the mix.
Do they have DBT therapy or workshops in the UK? We don't have very good resources here in So. California where I live, but my DD has a DBT workbook her T gave her. It is available online. Maybe that could help your son?
Let us know how this develops, okay, Dibdob59?
Hugs to you, your DS and your GS!