Try not to openly oppose him. It'll only drive him further into that religion. Instead get him to keep questioning why they do everything the way they do.
"Why did the translator of the New World Translation (their version of the bible) keep their names annoymous? What if I search them up?"
"Why isn't a bible study done with JUST the bible?" - Jw's will say the book is for help which contradicts the very reason why the bible teacher is there anyway.
I'm sure other people on this forum have better opinions than me. Just my 2cents