Sounds like he is full of bunk. Although you are in the UK, the petition is essentially the same here in the U.S. IT cannot be revoked, it starts the process.
You can follow-up with divorce proceedings after the "cooling off period" or you can not. There is no withdrawing the petition.
Regardless you need to be educated as if he was the petitioner and he doesn't file, you can apply for a decree absolute three months, six weeks and one day after the grant of the decree nisi.
Glad to know quacking sounds the same in the UK as it does here.