I agree with many of the posters, if he is 27 and already able to match scripture to scripture he is already fairly deep in. You need an alternative approach. I would go for the cult characteristics rather than doctrine.
I would say that you appreciate the conversations you have with him about doctrine as they are both interesting and stimulating and one of the things you really enjoy about the visit. Say as last time he introduced the topic of the Trinity could you perhaps pick the next topic for discussion for the next visit? If he agrees chose one topic and present your case clearly, calmly and succinctly. Perhaps 1914 and the generation. Cite historical verifiable sources that disprove dates. Deal only with facts rather than scriptual interpretation since the argument is so much stronger.
Another tack is to look up sources quoted by the WTBTs and show him what sources actually say. State that as the society has quoted from this source and provided details to source material in their publications then there can be no objection to looking them up. Look up some in advance that have been used inappropriately. If you can demonstrate that the society has been disingenuous and untruthful in some matters it may prompt him to start questioning what he is being taught.
The secret is to take it slowly and not frighten him off causing him to cut all contact by going in all guns blazing. Restrict your acguments to verifiable facts from respected sources. Keep it friendly and treat him in the same way witnesses treat a return visit. say: "I did enjoy this discussion and the points you made, as I picked this time what do you want to discuss on the next visit? I am really looking forward to our next chat."
It is hard to let someone down so although he may be disturbed by something you have said, human nature also wants to be agreeable so once he has agreed to a visit he is more likely to follow through. Then each visit have a goal in mind, keep it friendly and arrange next visit at the end.
I wish you well.