My cousin is marrying a Christian woman, how do I convince him marrying christian is strictly forbidden in Islam...?

Read the Qu'ran.  It's permitted.

No it’s not people of book

YOU DON'T CONVINCE HIM OF ANYTHING FOR JESUS IS THE WAY THE TRUTH AND THE LIFE. NO ONE ENTERS HEAVEN EXCEPT THRU JESUS AND ONLY JESUS CAN FORGIVE SINS.

Tell him about your prejudices and religious bigotry.

Both of them will have troubles in the marriage.

Show him the punishment now.

Islam beliefs lead a person to hell because islam beliefs reject the truths that the Lord Jesus Christ is God and Savior.  So get completely away from islam beliefs.  Here is the truth:  The Lord Jesus Christ is God.  Death leads to immediate heaven or hell. The only way to have eternal life, the only way to be in heaven and avoid hell, is by believing in the Lord Jesus Christ (John 6:47).  It is that simple and that easy.  It is too late to be saved after death.  Jesus is God and Jesus died on the cross and shed His blood to pay for all of our sins in full, and then Jesus was buried, and then Jesus resurrected from the dead (1 Corinthians 15:3-4).  The Lord Jesus Christ paid for our sins with His death and blood.  Nothing else pays for our sins, not works from us or anything else.  You cannot add any works to getting into heaven (Romans 4:5).

Question:  What kind of God wouldn't want more love in the world?  Answer:  a pedophile 

Tell him that he will suffer for all eternity if he does not cut off the marriage

it's ok if he's a male.