What were some events that changed Holling's character in The Wednesday Wars?

I think one "event" that changed Holling's character more than anything else was his time spent with Mrs. Baker.  Granted that's not one single event, but the time spent Mrs. Baker deeply affects Holling.  He learns to be a much more caring and supportive friend through her.  He learns what it is to empathize with other people.  He learns what it means to give selflessly of himself.  

One event that shows how Holling has...

I think one "event" that changed Holling's character more than anything else was his time spent with Mrs. Baker.  Granted that's not one single event, but the time spent Mrs. Baker deeply affects Holling.  He learns to be a much more caring and supportive friend through her.  He learns what it is to empathize with other people.  He learns what it means to give selflessly of himself.  

One event that shows how Holling has changed is when his sister, Heather, ran away and got stranded.  Holling's parents were not willing to do anything, so it was Holling that had to rescue his sister.  

I think another critical event that helped shape Holling was his run in with Mickey Mantle.  Mantle is Holling's idol.  It makes sense, because Mantle is one of the greatest ball players to have ever played the game.  It makes sense that Holling thinks he would be a great person too.  Unfortunately Mantle is a jerk and rebuffs Holling at one point.  Crushed, Holling returns home with his friend Danny, but not until after Danny returns his autographed Mantle baseball in order to show his loyalty to Holling.   Holling learns what true loyalty looks like from his friends, not some former idol of his.