Grim keeps calling Kevin (Freak) "poor Kevin."  Max does not understand why people keep calling Freak "poor Kevin."  To Max, Freak is anything but poor.  Kevin is smarter than most of the adults that Max knows, so it can't be poor in intelligence.  Freak is always rich with ideas, games to play, and adventures to have.  About the only thing that Max might consider "poor" about Freak is that he is small.  But to Max size shouldn't matter.  He's freakishly big, so somebody that is freakishly small doesn't bother him.  What Max does not realize at this point is that Kevin is sick.  I am not talking about a cold or flu either.  Kevin is terminally ill, and that is what Grim is referring to.