The previous post by Herappleness makes a good point, but I would still have to say it was the scene where Kevin comes to the rescue to save Max from his diabolical father, Killer Kane. Not only does Kevin save Max from kidnapping and possible murder, but it is his last great action of the story before his own death. The scenes with Killer Kane are the most serious of the novel, and the action builds to Max's father's act of abduction. The novel seems to be heading in this direction, with the occasional mention of Killer Kane and the death of Max's mom leading the reader to wonder if (1) Killer Kane will make an appearance; and (2) Will Max and his father reunite. Kevin's decision to rescue Max make for a heroic moment and a final act of friendship toward his huge pal.
It is tempting to award the climax to the intense scene when Killer Kane has gotten a hold of Max and held him tied up about to kill him only for him to be saved by Kevin almost miraculously.
However, I am more partial to say that the climax is actually when Max confessed or reveals rather, that he witnessed the moment when his father strangled his mother to death. That is a huge moment in the story that gives place to many other things to come to an end, and because it is the biggest 'wow' factor moment in the story.
I disagree with both of you. The climax is at the very end of the novel - the last page. The climax of a novel is the outcome of the conflict. They go hand in hand in the elements of plot. Yes, the climax is often the part of highest interest in a story, but not always. The conflict of this novel is that Max thinks he is dumb and slow. Throughout the novel he gains confidence, but it is not until the very end that he realizes his intelligence. It is when he gets the empty book out of the pyramid box and decides to write their story. The falling action is when he starts to feel okay about "remembering things". The resolution is his decision to possibly read a book or two. This also verifies that he now is feeling intelligent.