While the novel doesn't explicitly describe Freak's leg becoming unusable, it's implied that his condition affects his legs significantly. He uses crutches to help him walk and has difficulty with stairs and other physical activities. He also has a special wheelchair that he uses for longer distances.
It's important to remember that Freak's physical limitations are a part of who he is, and the novel explores how he overcomes these challenges to live a fulfilling life. His strength lies in his intellect, courage, and friendship with Max (the "Mighty" in "Freak the Mighty").