Is the mother's sudden illness after her son prayed for her to become ill, should he return to disobedience, due to his prayer?

Invoking illness upon your mother is forbidden and a clear act of disobedience. You must repent to Allah Almighty. It is not certain that her illness is a response to your supplication, as this is a matter of the unseen, but it is a possibility. The Prophet, peace and blessings be upon him, forbade invoking evil upon oneself, one's children, and one's wealth, fearing that it might coincide with a moment of acceptance [of supplications].

You must strive to be dutiful to your mother and frequently supplicate for her healing. You are also required to perform an expiation for breaking an oath.