[MadMoogle]

Could he have sued the studio if he could prove it was his idea?

It's been done before and is the reason why studios do not read unsolicited scripts. Art Buchwald sued Paramount when Eddie Murphy was given sole story credit for a movie he unsuccessfully pitched. Art won the lawsuit, but instead of appealing, paramount settled for $900k and in exchange the ruling was vacated.

[swimfar]

The movie was "Coming to America" (1988)

https://en.wikipedia.org/wiki/Buchwald_v._Paramount