The law appears to be clear that this fellow is not a US citizen. He was born in the U.K. to a UK father and a US citizen mother who had not lived in the US since she was 13.
From the State Dept website: https://travel.state.gov/content/travel/en/legal-considerations/us-citizenship-laws-policies/citizenship-child-born-abroad.html
Birth Abroad to One Citizen and One Alien Parent in Wedlock
A child born abroad to one U.S. citizen parent and one alien parent acquires U.S. citizenship at birth under Section 301(g) of the INA provided the U.S. citizen parent was physically present in the United States or one of its outlying possessions for the time period required by the law applicable at the time of the child’s birth. (For birth on or after November 14, 1986, a period of five years physical presence, two after the age of fourteen, is required. For birth between December 24, 1952 and November 13, 1986, a period of ten years, five after the age of fourteen, is required for physical presence in the United States or one of its outlying possessions to transmit U.S. citizenship to the child.)
Like Obama.