In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, the Fatwa Center at Islam Q and A, states:
A step-son is one of the maharim (non-marriageable men) of the woman. The fact that he is a Christian does not prevent this. However, if you notice a suspicious behavior from him, then you must treat him as non-mahram.
We recommend that you take care of this child, teach him Islam and encourage him to embrace it by your good treatment and so forth.
Almighty Allah knows best.