How can I invite a Jewish man who has previously embraced Islam to return to Islam, and what is the appropriate response to him, knowing my limited knowledge?

Calling to Allah is one of the greatest deeds. Calling the People of the Book focuses on clarifying that Islam is the true religion and presenting the proofs of the Prophet's truthfulness, peace and blessings be upon him, as well as refuting their beliefs that contradict Islam. If you are unable to explain this, seek assistance from students of knowledge and available resources. As for a disbeliever who insults Islam, he must be rebuked, shunned, and reported to the competent authorities. And whoever does not possess deep-rooted knowledge should not engage in debates with disbelievers.