Wow, you definitely have cause for concern! My thoughts are in line with what others have said - I wouldn't try to get into a scriptural battle over such a divisive issue. He is no doubt already being trained to expect "opposition", especially from family.
Instead, I might perhaps tell him that if he is truly interested in this religion, some research on its history would be prudent. After all, the WTS urges members of other religions to really examine their respective churches. The same level of scrutiny should be given to this religion that claims to have the truth. And if it really is the truth, then its history, teachings, actions, and claim to be god's mouthpiece would all stand up to that scrutiny.
I wish you the best.