He can’t prevent anyone that received the code under the GPL from using (and distributing it) under the old license. He also can’t relicense code that he received under the GPL only under the new license.
If he receives a new license from the other contributors to distribute under a more restrictive license, he can do that because he has a dual license to the code and is not relying on the GPL for his right to distribute.
Not true.
He can’t prevent anyone that received the code under the GPL from using (and distributing it) under the old license. He also can’t relicense code that he received under the GPL only under the new license.
If he receives a new license from the other contributors to distribute under a more restrictive license, he can do that because he has a dual license to the code and is not relying on the GPL for his right to distribute.