How can I invite a Christian to Islam and convince him, when he asks many questions about the religion and refuses to communicate with anyone else, claiming that I am a Muslim and should know about my religion?

You must continue learning to invite to Islam with clear insight, and be keen on inviting without delving into matters difficult to discuss so that the argument does not weaken. You must be wary of speaking about religion without knowledge and not be ashamed to say, "I do not know." It is good to clarify the virtues of Islam and the falsity of Christianity.