If memory serves, the dialogue doesn't state more than "with the return of the son, I can fix the body" (or something similar).
Upon watching the movie, I assumed that Mewtwo, despite all its power, was unable to fully generate a new body (organs and the like, which probably suffered a lot of damage in the car crash).
Thus, it would need Tim to provide body material (stem cells? DNA?); as Tim is Harry's son, maybe Mewtwo could produce/fix the new body with "healthy" bits of Tim. Kind of like in real life, some cells donations work better if they come from a family member.
Note that it also ties in nicely with the movie's overall "father and son finding each other again" theme.