Under Islamic law, since his father was Muslim, he is a Muslim. He says he is a Christian. Under Islamic law, that would be apostasy, which is punishable by death. Any Muslim who kills him will win a ticket to Heaven.
Of course he could be lying. This is not only condoned, but required of a Muslim if it furthers the cause of Islam. So if he's telling the truth, he should be killed, but it's O.K. if he's lying.
Keep this thought process in mind the next time you hear someone propose lasting agreements of peace, love, and dope with the Ummah.