EJP uses the contraction amn't all the time. I don't know where he could have picked it up, so I think he must have figured out the rules of contractions, and come up with it. It is a very reasonable assumption to make. There is "can not ==> can't" and "are not ==> aren't", so why not "am not ==> amn't"? I think this is the contraction of the future; just you wait.
He also throws forms of to do into his speech very regularly. For example, "I did want to jump out of the car," and "I do want to have a treat." To any linguists reading this, is there a term for this emphatic way of speaking? It's very cute to hear.