What is the ruling on one who claims to be the Awaited Mahdi, and does he thereby exit the fold of Islam?

It is not permissible to declare someone an infidel merely for claiming to be the Mahdi, because a mere claim does not fall under the categories of major disbelief that expels one from the fold of Islam. Many individuals throughout history have claimed to be the Mahdi, yet scholars have not declared them disbelievers. As Shaykh al-Islam Ibn Taymiyyah clarified, there were many who claimed to be the Mahdi, or whose followers claimed it for them, and despite their ignorance and error, they were in a better state than the Mahdi of the Rafidah.