And that is it, let your son hate you for a bit but I wouldn't tell him anything just yet about why, because he is probably very much brainwashed at this moment and will tell them what you said.
This is spot on, he is brainwashed, do not attack the cult.
I have had some very good advice from psychologist that I applied with my daughter, who has just turned 15, and it works, she no longer wants anything to do with the Cult.
The advice was, do not attack the Cult, do not try to draw the child into a loyalty conflict, offer you child an alternative. This offering the child an alternative is a difficult one to implement the other points are relatively easy.
I see in your case it is just before baptism, one suggestion
i would get a court order to stop it! it's for his safety
from GTTM seems good if you have the courage to do that.
He will hate you for it, but love you lots later. And with such things as a court order the Elders will back off.