Tell him the reason the deuterocanonical books aren’t in his bible is because Martin Luther threw them out. Early Christians accepted the deuterocanonicals because they were in the
Septuagint (Greek translation of the Old Testament) , and that‘s what the apostles used.
Tell him that no early councils endorsed the 66 books Protestants honor. The first council to address the issue of the canon was the Council of Rome in 382 under Pope Damasus, and it included all and only the 73 books we honor today. This canon was repeated at Hippo and Carthage (A.D.393 and 397), and has been repeated ever since.
Ask him, if for 1500 years the bible contained 73 books, and then a disgruntled monk comes along and throws out seven of the Old Testament books because they conflict with his beliefs, and then adds words to scripture (“only” in Romans 3:20; Romans 4:15, “alone” in Romans 3:28) to support his new doctrine on justification, wouldn’t he consider this monks actions a real ‘violation of God’s Word”—Revelation 22:8?
What’s more, the fact that he accepts the 27 books in his New Testament is a tacit admission that he accepts the authority of the Catholic Church on at least this one issue. After all, it was the Catholic Church that gathered together the books of the New Testament, grasped the Septuagint, and declared them to be the sum of Scripture. Did the Church have such authority? If not, why not add or subtract books from the New Testament as has been done with the Old?
Further reading:
Defending the Deuterocanonicals
Deuterocanonicals