The source code of Java (OpenJDK) is available for the public (at development time). It's not an officially stamped release, but you can use it to test your code.
Binaries with that have already been spotted in the wild. The source isn't. So I have no issue with Google calling it "open". "Open" has many meanings (e.g.: "'open' as in Office Open XML"). I just wonder what is it exactly they mean.
And no. He cannot fork Android H. The source has not been released. Others do fork released versions of Android to provide alternative ROMs (up to G). Not for H, I guess.