What is an important lesson will learns at the end of the book?

Once Will has accepted Zach's death, others begin to notice that Will is becoming more and more like the extroverted Zach. Miss Thorne wants Will to play Peter Pan in their upcoming show, but instead, he asks to play the flamboyant Captain Hook. He is a huge success and that night, with the applause still ringing in his ears, he realizes that he is truly happy and very glad to be alive. He realizes how far he has come. At home, when he hangs up his jacket on the peg, he finds that it is now too low for him and he announces to Tom - whom he now calls Dad - that finally he is growing.