This might depend on how confident you are that you can show him contradictions between WT teachings and the Bible.
Be especially careful not to get sucked into the WT yourself.
If you decide to give it a go, remember that questions work better than statements.
Don't try to prove anything to him; let him prove to you that the WT is right. After all, he thinks they have the truth and it's his job to teach you.
If you want to take a biblical approach, you might look for resources to help you.
I'd recommend Rhodes' book, Reasoning from the Scriptures with the Jehovah's Witnesses or Englund's book, Getting Through to Jehovah's Witnesses.
Both are available on Amazon.