It's more than just the way Maxwell resembles him, Grim says that night in the kitchen, the boy is like him, . . . "
The above quote is taken from Chapter One of Freak the Mighty. Max is introducing readers to himself and to his grandparents. Max's grandparents are Gram and Grim, and Max lives in their basement. The reason that Max lives with his grandparents is because his mom was brutally murdered. That's awful enough, but later in the book, the reader discovers that the murderer was Max's father. That's who Grim says that Max is starting to look like. Max looks just like his father, which is not a great thing for Max's already low self esteem. Max is a big kid, so he sticks out that way. Plus, Max isn't that smart, so he stands out at school for being a terrible student too. All of that adds up to give Max a very low opinion of himself. He calls himself a "butthead" multiple times in the first chapter, so looking like his murderous father is just one more knock against poor Max.