Ask him why Islam refuses to subject the Koran to historical-critical and literary-critical scrutiny.
Ask him why the Koran affirms certain things about Jesus (e.g., that Jesus never died on the cross), when many texts from earliest Christianity (see above, for example) affirm just the opposite.
Not only from earliest Christianity, but eye-witness accounts (Matthew and John) to boot. How can the Koran seriously refute eye witness accounts?