I've been thinking about that. I researched a little, and most of the experts (in my quick search) said narcissists are rarely prone to suicide. But, prison is a different story.
If he's in solitary, he won't be able to "hold court." I think it may be difficult for him to cope with not being the center of attention. If he has to be in solitary for any long period of time, he may commit suicide.
If in the gen pop, he may be liked as an affable goof-ball and get along ok and be affirmed enough by inmates with low self esteem (and he knows how to groom and manipulate this group well, along with fellow-narcissists, who apparently are well represented in prison), or he may be disliked for being arrogant and mouthy, and be a target. Maybe both at the same time. I see him as being a schemer and manipulator of fellow inmates and guards -- he'll always have something up his sleeve.
Not quite sure what he'll do about his apparent sex addiction, unless he's willing to play on the other team.
He, will, of course, consider himself the victim. And rage against the system for putting him into prison unjustly -- after all, he's above the law.