If he changes all art and story, he can release his game. Copyright on video games only protects the actual code and the art. Not the idea. (It actually doesn't even protect the title)
If he released it AS-IS with the models so blatantly based on Among Us, a title also blatantly based on Among Us, gameplay blatantly based on Among Us, and a map he openly admits to having ripped directly from the original game, he'd be in trouble. And while not protected by copyright law, in many places common law trademark protection would apply to the title.
But yes, using the game mechanics on his own maps and with his own character designs and a less blatantly referential title would be entirely legal. It would be COOL if he can get permission from the devs to release it using the assets based on their work though. So I hope he does that.