Okay, so he's been body-swapped, but what about his personality? What kind of themes are you grappling with? How old is he?
Some common sad backstories are: Death(s) of family member(s) or romantic interest, parents getting divorced, losing your job on the same day as your girlfriend dumps you and your house burns down which also happened to be your birthday, your favorite TV show ended prematurely, your crowning achievement was a failure, you're middle aged but you haven't achieved anything important, you're middle aged and successful but your elderly father still doesn't respect you and you're still not happy.
The possibilities are endless but I can't craft anything personalized unless I get details to work with.