The Other Son [Movie}

This is a movie I will watch (& I don't watch too many).  The two boys in "The Other Son" have been raised to resent and hate each other, and now find out that they are each other. The "Jewish" boy is not Jewish, the rabbi explains to him, because his mother was not Jewish. The boy protests that he has been an observant Jew for every day of his life. Not good enough, the rabbi regrets. The boy has only to take a few more steps and he can convert. The boy is outraged--he, a lifelong Jew, must convert to Judaism?