You may speak to him if you are doing so in order to convince him of the truth of Islam through persuasive arguments. People may fall into wrong ideas because of ignorance or due to being brainwashed by islamophobes.

Therefore, if you believe you have the ability or expertise to deter them and bring them back to the fold of Islam-- you may dialogue with them. If, on the contrary, you are not so confident, and rather you are afraid of losing your own faith in the process, then you must simply walk away from them, saying 'peace'. Allah says: "The servants of the Most Gracious are [only] they who walk gently on earth, and who, whenever the foolish address them, reply with [words of] peace." (Qur'an:25:63).