Does a specific individual who doubts that Allah has a companion become an infidel, knowing that he has not heard of the Qur’an, and does his doubt necessitate his excommunication, or is it until proof is established against him?

One cannot be a Muslim while not believing that Allah the Exalted has taken neither a consort nor a son, because this contradicts Allah's Oneness, perfection, and self-sufficiency. Belief in this is a fundamental principle in Islam, and doubting it is disbelief in Allah. There is no excuse of ignorance in this matter; rather, one who is ignorant of it disbelieves, because the proofs for it are manifest and clear.