Trying to remember a book when I was a kid. Dragons, Kid named Brian (?) uses potato slices as weapons...?

It was a book for probably 10-12 yr olds. I think the main character was a kid named Brian but I could be wrong. There were dragons attacking the world and he entered their world to save it. I remember he whittled a dog figurine, lost it, then a dog just like his figure showed up. He used a special knife to do the whittling, and used it to slice potatoes that hardened into stone and were weapons toward the dragons. Anyone know who the author is? What is the title?