Well, the first difference is the title: the book Freak the Mighty becomes the movie The Mighty. This shows that there are other changes as well. Much of the movie follows the book fairly well, until the ending.
At the ending of the book, Kevin has another seizure on his birthday. This is the seizure that puts Kevin in the hospital for the last time. Max visits Kevin and, during that visit, asks Max to write down their adventures. Kevin dies in the hospital after Max visits (and after Kevin has yet another seizure).
At the ending of the movie, it is the Christmas holiday that brings the boys together. Kevin actually gives Max a blank book in order for Max to write their stories. Instead of dying in the hospital, Kevin then dies in his sleep due to his heart issues. When Max learns that Kevin has died, Max chases the ambulance. This is how Max ends up at the hospital in the movie.
In conclusion, the movie and the book do end the same way: Max writes the adventures of “Freak the Mighty.”