Let him add 'not' to the mixed up ones. Then let him do all the work, deciding who will be mixed up. It's okay if he'd rather get them wrong on purpose. This is Daddy. Let him read it, so you know he did it on purpose.
Silly games are more fun for a lot of kids.
Later, you can add other sentences if you want. I am and I can are good starters that let you add new words. William is a boy. William is three years old. William is not silly.