That is for him to find out. When my husband was diagnosed with NPD by a doctor, I was told, not him. As you well know, you can't tell them anything., because they are always right and nothing is ever wrong with them. You never know how he might react. Generally, they are a cowardly lot, and you don't have a lot to worry about.
Separation is no fun.
And no matter how quietly you think you are going to do it, be prepared for some aggro. Important to have a good lawyer, and to keep things as low-key as possible. Ensure that you get your practicalities in order.