The fundamental question is "Does he believe that Jesus is God made Man, and the Messiah foretold in the Bible?". If yes, then he is a Christian, and Muslims will not accept him as a Muslim because it is part of Islam that God has no son.
There is not such a clear, scripture-based basis for saying that a Jew who believes that Jesus was the foretold Messiah is no longer to be considered a Jew.
Question again: do you think it's possible to be atheist and still Jewish? There are plenty of people around who are atheist, of Jewish descent, and are still considered Jews by many Jews.
It is possible to be atheist and Jewish if you accept the idea that Jewish is not just religion-based, which many do. There does seem to be a genetic component to being Jewish, and that is a disease more prevalent to Jews.
My mother, and many others, view Judaism as religious, cultural and genetic, but the most important aspect remains your belief system and that system totally rejects Jesus as messiah.