Please please please know that I do not speak on a professional level. In fact, I will change my profile to state that I am not a counselor. So, my posts on psychological matters are merely from personal experiences or opinions .. mostly based on personal experiences.
This friend does not sound as if he has multiple personalities. For discussion purposes, I'll say he sounds as if he has narcissistic traits. I gather this by reading this:
He has told me he is somewhat aware of this, but I've gathered he believes it to be normal, and once told me it is because he has will enough to change personality traits depending on his moods, which made me worry more really.
Narcissists are very intelligent people - or appear very well to be! They can in fact change themselves to be whatever they need to be in order to achieve their goals. They don't dwell on this as a flaw - they see it as a necessity.
If confonted with these "troubles," they may be very defensive and may bark back at their "attacker." The enemy is someone who tries to show them that their narcissistic behavior is a disorder, or that something is wrong with them. They cannot admit that something is wrong with their behavior. They have deep issues with self esteem and no one but they can know about it.
Anyway, that's my take on narcissism and I only say that based on what I got from your post. So please do not take my opinion and run with it -- like I said, I am no professional and am not trained in these areas. I've just done my own research on such things. My thoughts on your post are purely for discussion purposes and in no way intends to help you identify his problems.
In any case, Stan is very right. Having a label for his problems will not help either of you find solutions to them. And yes, if you give him the idea that something is wrong with him, it might just blow up in your face. If you and he are talking about his issues and he welcomes your concern, the best thing would be for he HIMSELF to seek counseling. Studying psychological disorders or illnesses is not the same as being trained and equipped to handle someone's mental and emotional stability. You might end up hurting him or yourself in the process.
If, in time, you feel as if he may harm himself or others then that is the time to step in and obtain help for him.
All that said, welcome to the board!! I hope you stick around and chat with us!