Remind him of how the society and God view those that make false accusations. Make him agree to prove you are telling lies before he can label you as an apostate or liar. You also need to agree on what the definition of an apostate is. Make him agree that someone who tells the truth in a matter can't be an apostate.If you are going to discuss the societies claim of divine inspiration, you need to agree on the definition of a prophet beforehand. Don't wait until you are in the middle of a discussion and then try to agree on the definition. Do it first before you talk about anything. Write it down if you need to so that he can't go back and say, "I didn't say that."
No getting mad and walking out either. "If you want to know what I have to say, you have to stay until I am finished."
Excellent. Ask him questions that require a Yes or No answer. Tailor these so that he either confirms the bible is true or false with his answers. In doing so you may get him to see that if he believes the bible on a subject which contradicts WT dogma then the WTBTS is flawed.