Who informs Congress of the person that cannot carry out the duties

President himself informs Congress of the person that cannot carry out the duties.

Explanation:

If the person holding the post of the President finds himself incapable of performing his duties as the President due to any given reasons, he informs so in writing to the speaker of the US Congress and also to the pro tempore president of the 'Senate'. During this period of 'presidential disability', the Constitution authorizes the Vice-President to carry out the 'duties' of the President from his office.