Just off the top of my head, you could ask him where does Jesus state that his followers need to "be in" ANY organization to be pleasing to the Father?
By what authority does the WTS say that leaving them is the same as leaving God?
IF Jesus thought it was absolutely imperative to "use the name" of Jehovah (as the WTS says they must)....WHY didn't Jesus "use" the name when he instructed his followers to pray the model prayer? Wouldn't THAT have been the most appropriate time to bring that out---if it was really a bible teaching?
How can the WTS arbitrarily choose which bible terms and positions apply to THEM and what applies to "Christendom" who they despise? It all seems to boil down to what THEY DECIDE applies to them! No proof is needed!
Ask him to show you in the bible, where Jesus chose the WTS as his favorite religion in 1918.
If "Christians" who speak the "pure language" are not supposed to celebrate ANYTHING but the Memorial because they might have "religious connotations" attached to them....then why are Mother's Day and Father's Day, etc, etc, "displeasing to Jehovah" and bad to celebrate? What "religion" are they honoring?
Why does Jesus command that his followers be baptized in the name of the Father , the Son and the Holy Spirit.....but the WTS has taken the liberty of baptizing ITS followers in an entirely different manner, which changes every few years? Isn't what JESUS SAID good enough for the "Faithful & Discreet Slave" to follow?
Just a few for starters if you need them......
Best of luck with him,
hugs,
Annie