Your question is confusing simply because you do not mention which part of the book to which you are referring. As a result, I have chosen to concentrate upon Max’s absence from school due to Kevin’s death. This happens near the end of the book.
Of course, Max and Kevin have become inseparable friends. As a result, Kevin’s death is a devastating blow to Max. Max has to deal with the truth: that Kevin made up the story about having a “bionic body” in order to deal with his severe physical condition. Max learns that, as a result of Kevin’s birth defect, his heart has literally gotten “too big for his body.” As a result of Kevin’s death, we learn that Max's retreat into his “down under” doesn't allow him to go to school because Max stays in the basement “for days and days.” Therefore, Max is not going to school due to his loss. Max says that he feels like “a balloon somebody had let the air out of.” Grim and Gram treat Max with compassion in regards to his grief; however, they eventually have to threaten to “drag” Max back to school.