In the name of Allah, the most gracious, the most merciful He is not considered as Muslim unless his faith is Islam... and Islam only. May Allah guide you to please him.
Just denying the Imamate of Imam Musa al-Kadhim (peace be upon him) does not make the person an enemy of the Imams (a Nasibi). However, if such denial consists of hostility, then based on an obligatory precaution it is not permissible to marry him.