JWs are programmed to teach. Not to be taught.
What you should do, is ask him to prove to you that the fall of Jerusalem occurred in 607, WITHOUT using WTS literature.
This is how I approached the subject.
The fall of Jerusalem happened. Every one agrees on that.
Ask your son: WWII happened when it happened, right? You could live anywhere and it still happened when it happened. You could be a Baptist, Catholic, JW, Athiest, and WWII still occurred on the date it occurred, right? It is a matter of fact, not speculation, right?
Could you please find some solid references from historical scholars outside of the WTS that support the fall of Jerusalem occurring in 607? (PLease do not give me literature from the WTS, because that is like saying, "It happened because we say so.") This is a matter of historical fact, so it should be proveable. HOw did the WTS arrive at this date?
Then do the backward count of the 5 kings who ruled and show him how you are arriving at 587.
PM me, if you need that info. I have it. It made BIG waves with my JW friend.