Kevin, also known as Freak, suffers from Morquio syndrome. This is a rare disorder that causes all sorts of health problems. As a result of this disease, Freak's body has not grown properly. Among other things, his limbs are too short, and he has to walk with the aid of crutches and braces.
Although it is later revealed that Freak knows that his condition means he will not live into adulthood, he tells other people that he simply needs a new bionic body. In chapter 14, Max asks his friend why he needs a new body. Freak responds that everyone's body changes, but his body just won't grow on the outside. Freak is acutely aware that the truth of his condition may be too much for Max to handle. That's why he tells him that he just needs a new body, and then everything will be alright. Max believes this story. Perhaps deep down he knows that there is no such thing as a bionic body. However, he finds it easy enough to believe because the idea that his friend is dying is a lot for a young person to handle. That is likely why the terrible news of Freak's death hits Max so hard.