I need help remembering the name of a book/book series?

I don't remember much about the book. I do remember that it involved a young boy, who may have been mentally handicapped in some way. He gets a chihuahua and pierces it's ear. I also remember that he sticks his finger into a pencil sharpener to try and sharpen it, and he also accidentally cuts off the tip of a clmates nose. I think that it also may have been a series. In one book he is with his grandma who needs an oxygen thing to breath with, and she sends the boy to get her cigarettes. And I also think that they go on this field trip to visit an amish house and he eats an entire shoe fly pie or something. I really would like to read this series again, but I cannot remember the title of the books.