Actually no, as neither parent was an American citizen when he was born. Furthermore, Boris was never recognized as a U.S. citizen. He was born on June 19th, 1964 in the Upper East Side of Manhattan, New York City, and his family moved back to England in Sept. of 1964. He has never applied to become a U.S. citizen either. Thus, he is not a citizen, let alone a natural born citizen.
As neither parent was a diplomat, BoJo was a US citizen by birth. He renounced his US citizenship in 2017.