Your best bet is to attack the doctrine of the Faithful and Discreet slave. They say that they are the Faithful and Discreet slave Jesus spoke of in Matthew and once you challenge that doctrine just about everything else crumbles. Without the FADS the authority of the religion is taken away from them, so you could just worship God in the way you want, not them.
Ask him if he thinks it is logical to believe in a few dirty old men sitting around a table in Brooklyn that have had no knowladge of living in the real world and listen to them rulling the lives of millions of people.
Is it logical to believe Jesus really selected a group of spiritualistic miracle wheat eating young and naive men to be his spokesmen on earth? ( www.escapethewatchtower.cjb.net )
Is it logical to believe that God would change his 'unmistakable truth' so many times to fit in with what they say?
If its logic he is after, tell him to convert to Islam and bow several times to the east whilst worshipping God, as surely God would prefer this method of worship other than a hundred plus braindead pupils muttering the thoughts of 12 dirty old men repetitively in polyester suits.