Tell him the truth, though all men be found liars, like Bethel's current leadership, let God be found true. There is the Almighty God forever, human corruption is endemic since Eden's sin, God provided PROMISE of the seed of recovery (Gen3:15), and that is still TRUTH, the truth. God tracked all that development of sovereign rival power, and his own Kingdom forming power through Daniel and to the apostle John and all the prophets for a good 2800 years of prophecy so accurate, it is now called world history.
Jesus Christ, the seed, the true solution, the first life created by God, from God, is also still true and the only true hope to retrieve men past, present and future from inevitable death in Adam. That is all still true, neither Christendom, Bethel or any other human organized diversion from truth can change reality that is actually true.
Thus tell your son the truth. Bethel's primary leaders are now corrupted, they joined the opposing kingdom power as UN NGO, that will bring a judgment on JWs for that transgression (Dan8:13 as 1Pet4:17), and they will reap what they have sown, it is also foretold in prophecy they cover up (Dan8:12b) and truthfully, they no longer represent the truth (2Thess2:10-11), instead they drive millions and millions AWAY from God, but God, as before on several documented occasions of prophecy again becoming history will recover the final truth, again (Rev10:11; 2Thess2:8).
You may get to the core of the dilemma, ask your son does he want to be taught about God and Christ by lawless corrupted men in the so-called Governing Body?
And I feel you can ask that question quite honestly while still maintaining core Christian truth, that truly at one time, 1914-1950 IBSA and JWs did at once truthfully represent as best they could, and they did expose the first 2 UN related fulfillments of prophecy in 1919 and 1945, rather than joining the UN, and covering up the 3rd (1990) and 4th (future) UN manifestations of Daniel 11:30-45, as Bethel also covers up King North as 8th King developments there, as their continual UN NGO internal Bethel endorsers are also prophetic subverters.
You may explain, truthfully, Jesus Christ warns Christians to flee the city (organization) once one personally ascertains the rival kingdom "disgusting thing" in it's midst, and in principle that applies to Bethel as UN NGO and the JW organization overall:
(Matthew 24:15-16) “Therefore, when you catch sight of the disgusting thing that causes desolation (UN NGO), as spoken of through Daniel the prophet (1990 Dan11:31b parallel to Dan8:13-14), standing in a holy place (Bethel's claim), (let the reader use discernment,) 16 then let those in (JW worldwide) Judea begin fleeing to the mountains. (leave JWs, judgment is nigh).
In my opinion explaining these core truths of Bethel's corruption are important, and outweigh all their other sins (2Thess2:3-4) in direct guaranteed import of a judgment to come first upon JWs; (1Pet4:17).