What is the ruling on dealing with a person born to Muslim parents who has declared his atheism "as a form of intellectual showmanship"?

This person is in grave danger. The ruling concerning him is that he is an infidel if his heart has opened up to disbelief. It is obligatory to strive for his guidance and to convince him of the necessity of adhering to Islam and the peril of atheism. As for speaking and dealing with him within the bounds of the Sharia, it is permissible.