C. Moore--Tell the man that the Encyclopedia Brittanica has more on the life of Christ that it has on the life of Shakespeare. He has no doubt about the existence of Shakespeare primarily because he has no antagonism to the existence of that one man.
He has a psychological problem with the existence of Christ, however, and that phobia is his real problem. Let him know that if you put him in a roomful of books from secular authorities that discussed the secular records of Christ's existence, it would do him no good. The only faith that is real faith comes from the word of God. The Holy Spirit uses it to convert men to Christ. It is our job to bear witness of Christ. If the Holy Spirit is pleased to use it to give that man a REAL faith, He will do so. If He (the Holy Spirit) is not pleased to personally convince that man through the testimony of Christ in His word, nothing else will help him. We bear witness through the word, and have no interest in secular testimonies, nor do we need them. BUT, if you are personally interested, Josh McDowel gives a number of different secular sources in one of his "Evidence That Demands a Verdict" editions. I know that Josephus, a Jewish priest, wrote The Antiquities of the Jews and made a reference to the Christ in that history book. The Ency. Brittanica is probably careful to give footnotes and a bibliography of ALL sources it used it writing its article concerning Christ. That may be the largest listing of sources.