I would ask him if HE beleived the bible...then i would ask him if jw`s beleived what the bible says , if their teachings were based on the bible .
He no doubt would answer yes of course to those questions . Then point out the bible has been the same for almost 2000 years and been used by catholic, protestant, and eastern orthodox religions during that time.
Then point out beleifs , one or many ,of your choice where jw`s have changed their understanding of certain scriptures that has radically altered their beleifs.
So my point is they dont beleive what the bible says , they beleive in the interpretation the wt gives to certain scriptures , which is in constant change over these past 130 years .
Their is a big difference in beleiving what the bible ACTUALLY says against beleiving in an interpretation of what you THINK the bible means.
Just my 2 cents worth