How does the fact that Max refers to his grandfather as "Grim" contribute to the tone in the novel?

I have never thought twice about Grim's name until I read your question.  I just figured that was the nickname that Max called his grandfather.  He calls his grandmother Gram, so Grim seemed to fit.  My own kids call grandma "geema," even though they are quite capable of saying the word grandma.  

But upon thinking about the word "grim" in relation to the novel, the grandfather's name takes on an entirely different meaning.  The...

I have never thought twice about Grim's name until I read your question.  I just figured that was the nickname that Max called his grandfather.  He calls his grandmother Gram, so Grim seemed to fit.  My own kids call grandma "geema," even though they are quite capable of saying the word grandma.  

But upon thinking about the word "grim" in relation to the novel, the grandfather's name takes on an entirely different meaning.  The tone and mood for Freak the Mighty is dark and depressing.  Max himself is not a happy character.  When the reader is first introduced to him, Max repeatedly refers to himself as a stupid butthead.  He lives in a dark, depressing basement which he calls the "down under."  His father is in jail for murdering Max's mother . . . in front of Max.  Many of the plot elements and feelings that Max has are uninviting and gloomy.  That is the very definition of the word "grim."  By naming a character Grim, the author is able to further sell the novel's dark tone.  

Other authors have done the same thing in other novels.  For example, the book Chitty Chitty Bang Bang by Ian Fleming is lighthearted and fun through most of it.  Character names reflect that tone; most notably is the character Truly Scrumptious.