Here are some great movies with themes similar to those you can find in Of Mice and Men (not repeating any of the good ones already mentioned):
Wonder: Auggie was born with a rare facial condition which has required 27 surgeries, and he has been homeschooled his entire life. In this movie (also a great book!), he attempts to integrate into the social structures of his local public school. The themes of finding acceptance with differences is similar to Of Mice and Men.
Rain Man: This movie features two brothers on various adventures. Raymond, the older brother, has autism and his younger brother, Charlie, decides to use him and his keen memory abilities for his own advantages. Much like Lennie, Raymond relies primarily on a man who doesn't always have his best interests at heart.
Radio : Based on the true story of a man in South Carolina, Radio is an intellectually challenged man who is treated unkindly by the local football team. The coach witnesses this and invites Radio to work with the team....
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