It would be correct if you had a legal contract granting you rights, but as it stands, he owns the game's copyright and everything that comes to it since he is the creator and owner.
You were only brought in as a writer, meaning, you own nothing (I'm truly sorry, but that's how it is.)
Technically speaking, he could legally force you to release everything you've got and he has no access to, as it is his property.
Partly correct, they're complex and certainly designed to help out the bigger ones. Which in your case, is 100% against you, as you are not the owner
I really am sorry, this situation sucks, especially since it was a really good game.
But in my honest opinion, you'd be much better of creating something else, while similar, but keeping it different.
I mean, I do not know what the current situation is with the game's owner, but if it's still as bad, it would save you a lot of potential issues later on.