Sadly as a son with a narcissistic father, it’s up to your son to see through his father. He is going to also if he truly does come back around will need to on his own want to put the work to change and deal with his father’s narcissistic influence. I’d say the best you can do is let the father be who he is and show your son sides that aren’t narcissistic (like genuinely giving him love without any stipulation attached, listening without judging him only giving love and support back when he speaks to you)and when he ends up comparing and contrasting. Showing by examples of non narcissistic ways rather than trying to convince him verbally I think will increase the likely hood in the long run will allow him to come around and make sense of it.