article II, section 1, paragraph 1 of the Constitution of the United States of America:

The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice President, chosen for the same Term, be elected, as follows:

It’s not even in an amendment, it’s defined in the original text. No way he gets enough states together to ratify an amendment to increase his term length.