A woman left an anonymous comment asking for advice because her son's father is a narcissist.
Readers...please feel free to share your thoughts.
I suspect one could spend a tremendous amount of time and energy trying to limit your child's exposure to the toxic ex-spouse, but due to the custody agreement, that might not be possible. (If your ex is anything like the Michelle Pfieffer character in the movie White Oleander, he'll relish any opportunity at child-free time.)
If there's not much you can do in that direction, then I'd do everything I could to provide a safe, loving, nurturing haven for your son, a haven where he is free to express and explore his feelings, to develop his identity without manipulation and guilt, a place where he can recuperate and regain his sense of self...a place where it's all about him...where he can be respected.
I'd read all I could about the art of "mirroring" your child. For example, if he cries because he fell, instead of telling him to quit crying and that it's just a little boo-boo when it's a broken arm (not that you would!), say that the fall must have really hurt and carefully inspect the damage. If he says he hates his first grade teacher, nod and ask why he feels that way. Sorry. You probably know what mirroring is but, just in case.
I believe it was the psychoanalyst Alice Miller, author of Drama of the Gifted Child, wrote somewhere (of course, now I can't find it...damn) something to the effect that the impact of a narcissistic parent could be offset by another parent, another member of the family, a family friend. I found this an extremely hopeful and powerful statement. You may not be able to control your toxic ex-spouse, but you can try to become the best parent you possibly can by practicing mindful parenting.
Sliv in a comment wrote that the father of her young child lies, steals and cheats to get his way. I have to admit, I have no direct experience with that kind of diabolical/malignant narcissist, but I suspect that as the non-narcissistic parent, you'd almost have to go into mirroring hypermode to counteract the damage.
Having never had a parent who mirrored, this was something I had to learn how to do with practice, finally achieving some level of competence just in time for the fun teenage years, when you're just dying to give advice and that's the last thing your kid wants to hear (five words strung together constituting nagging). So even if you have the lousy luck of having narcissistic parents and, maybe not surprisingly, a narcissist for a spouse or ex-spouse, it IS possible to improve one's parenting skills by educating ourselves and making a mindful effort!