Freak's explanation for a robot body is that he is participating in ground-breaking medical research which will make him the first 'bionically improved human.'
When Freak shows Max the location of the 'Fortress,' he tells Max that he has been going to the medical research facility for tests every few months and that eventually, he will be fitted with a new bio-engineered body. He goes on to explain that he will be fitted with a 'whole new body...only enlarged and improved.'
Later, Freak tells Max that no one stays the same and that things sometimes change for individuals. He mysteriously lets on that he is 'growing on the inside but not the outside.' After Freak's death, Dr. Spivak admits to Max that Freak had possibly invented his story about a new robot body because he 'needed something to hope for.' She explains that Freak died from complications due to an enlarged heart.