First off, I agree with LeavingWT. Facts mean nothing to a JW. The power of belief will outweigh the power of fact.
But if you insist on discussing it with him......ask him to show YOU....using material not published by the Watchtower....that Jerusalem fell in 607. If he insists on using WT material that is still ok because you can start by agreeing on one major thing. That Babylon fell in 539BC. Once you two agree on that:
1) Insight Book - Vol 2 - p458 "Nabonidus is believed to have ruled some 17 years.....Nabonidus' ascension to the throne followed the assassination of Labashi Marduk."
2) WT 1/1/65 p29 - "Neriglissar, who reigned 4 years"......"his underage son, Labashi Marduk, a vicious boy, succeeded him and was assassinated within 9 months"
3) WT 1/1/65 p29 - "Evil Merocach reigned 2 years and was murdered by his brother in law, Neriglissar, who reigned 4 years".
4) Insight Book - Vol 1 - p773 "The Babylonian King who succeeded Nebuchadnezzar to the throne" (this is under the heading of "Evil Merodach")
5) Daniel Book p 50 "Nebuchadnezzar, who reigned for 43 years...."
*So....
Nabonidus - 556BC-539BC (17 years)
Labashi Marduk - 556 BC (nine months)
Neriglissar - 560-556 BC (4 years)
Evil Merdach - 562-560 BC (2 years)
Nebuchadnezzar - 605-562 BC (43 years)
**According to the Insight Book - Vol 2 - p481 "in the 11th year of Zedekiah (which was Nebuchadnezzar's 19th year of reign including his accession year/18th actual REGNAL year), a breach was made in Jerusalem's wall.
***If Nebuchadnezzar began ruling in 605 and you subtract 18 years....you come to 587BC
All of this....using ONLY WT material