He must make up the fast and atone for it; because the testimonies are for proof, not for denial, so the testimony of the one who affirms is accepted. This is unlike the case where two testify to the sunset and two against it during Ramadan, and he breaks his fast and then it is confirmed that it did not set. See the acceptable comments and the Al-Hadiyah Al-‘Alaiyah, p. 166.