He's not dumb and he's not oblivious, so I'd be weary about the no sense of self thing. Charisma is all about you and how other sentient beings react to you.
I would play him like someone that no-one respects, even if what he says is right. You know the type:
Cha4:"Maybe we should check the door for traps..."
Warrior: (ignores Cha4) "Rogue, do you think there's a trap on the door ?"
Rogue: "Good thinking Warrior, let me check." (proceeds to search for traps)
Cha4: "But it was *my* idea..."
Wizard: "What are you talking about ?"
Cha4: "I mean... I suggested that we should check traps."
Wizard: "The *Warrior* said that."
Warrior: "Yeah, I just did."
Cha4: "I mean, before you said so."
Wizard: "No you didn't."
Warrior: "Stop being such a jerk. You wonder why people don't pay attention to you. If you weren't so annoying all the time, people would respect you more."
Cha4: "But it's not fair, I just..."
Rogue: "Would SOMEONE please just SHUT HIM THE HELL UP already. I'm trying to concentrate here."
Wizard: "Shut up, or you'll make Rogue spring a trap and risk all our lives, like last time."
Cha4: "But it wasn't me..."
Warrior: "SHUT UP!"
Cha4: *pouts*