Muddle Earth (2003) from Paul Stewart maybe?
Book's back cover, courtesy of Goodreads:
Joe Jefferson is an ordinary schoolboy from ordinary Earth. At least, he was. But something strange happened when he was walking his dog, and now he's Joe the Barbarian—fearless warrior-hero, summoned by Muddle Earth's leading wizard* to slay ogres, wrestle dragons, and bravely confront villains.
Joe doesn't feel much like a warrior-hero.** But evil is stirring in the heart of Elfwood, and the people of Muddle Earth need help (although most of them don't know it yet). Perhaps Joe Jefferson really is a hero after all...
* Actually, Muddle Earth's only wizard. And he's not very good.
** He doesn't really look much like one either.
It's a very funny parody of Lord of the Rings. At the beginning of the book, Randalf opens a portal on Earth, and Joe's dog goes right through it, so Joe follows him and ends up in Muddle Earth.
As for the saucepan, at some point he goes with the (fraud) wizard to a city, to be "equipped" for his future tasks as a knight/warrior/whatever. He has high hopes, seeing shiny armors, but the wizard's broke so they look through hand-me-downs and get him a saucepan as his helmet: