Can a Christian be a Moslem if he practices elements of both?
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.