Is the child considered illegitimate if the marriage from which he was conceived was void and consummated, then the wife was divorced while pregnant and subsequently taken back, and the nullity of the contract was discovered later?

It is astonishing to witness irresponsible behavior from those living in Muslim lands, and the necessity of asking questions before embarking on any action, especially in matters of marriage. What you have done is not a valid Islamic marriage; rather, it is a form of adultery (zina). You must sincerely repent to Allah. If you had intercourse with her believing the marriage was valid, then the child is attributed to you. There is no divorce or reconciliation in this case because what occurred was adultery. It is mentioned in the hadith that "the child of adultery is the worst of the three" if they act according to their parents' deeds.