Austin's main struggle in Mean Boys is definitely Man vs Self. More precisely, Man vs Anxiety. The other characters' conflicts are all Man vs Self too, now that I think about it; I guess it's a staple of coming-of-age stories.
(Also, I just really like that this type of conflict lets you have the characters screw up until they hit rock bottom and then pull themselves out of the mess they made, with or without the power of friendship or love.)