Only part Jewish? Which part of the Torah do you believe then?
If his father is Jewish then he is Jewish. If not he isn't. Which is why the Bloodline comes from the father and not the mother. I realize this is a very contraversial subject to some but the science proves that the bloodline comes from the Father and Scripture confirms that. If it were through the women it would mean the 12 Tribes of Israel were not Jewish. Of course we know the 12 Tribes of israel were Jewish which again confirms the bloodline comes through the Father. This is why Jews are called the sons of Abraham, Isaac and Jacob instead of Sarah, Rebecca and Rachel. Both Rebecca and Rachel were Gentiles. So was the mother of Boaz and Obed. Note* the Halacha was written by sages and is not found in the Torah.
I have a friend whose father is a Jew and his father was a Jew but the mother of my friend is a Gentile. It was quite a discovery to him to learn he was a full blooded Jew as he has been told his entire life he was only "part Jewish".