Another approach that can be taken is to bring up the issue of the copper serpent in the wilderness. The mosaic law forbade the making of idols and being induced to serve them or look to them for salvation. Yet later, God brought a plague of poisonous snakes on the Israelites because of their disobedience. He then instructed Moses to make the graven image of a copper serpent and instruct the people to look to it to gain salvation from the venom of the poisonous snakes.
Most JWs will readily admit that what the Israelites were doing with the copper serpent would, under normal circumstances, be regarded as a form of idolatry but since this was a command of Jehovah that they were following it was legitimate - a God sanctioned exception to the general rule against idolatry. This is what you jump on. You mention the fact that Jesus himself, in the gospel of John, mentioned that this copper serpent's role in saving the Israelites prefigured his role in saving all from sin.
If God made an exception to his rule against idolatry to use the idol of a copper serpent to save the Israelites, how can JWs be sure that God hasn't also legitimized the worship of Jesus as being an exception to his general rule against worshipping others? And especially since the copper serpent prefigured Christ.
It can also be mentioned that the reason that worshiping God through idols is forbidden is that no idol can truly represent God. But this is not true of Jesus since Hebrews 1:3 describes Jesus as being the exact representation of God's very being. This would mean that, quite unlike any human-made idol, Jesus does qualify as an accurate representation of God and therefore worship of God through him - i.e. using Jesus as an "idol" - would be legitimate. Then you can tie in the scripture where Jesus said he that honors me honors the father and make the logical extrapolation that that would also inescapably mean that he that worships Jesus is worshiping the Father since worship and honor, according to JW theology, differs from each other only in degree.