I agonize about this for you. But of course it does no good. You are the only one who sees all the nuance, and even you can't see it all till you experience it.
I will say this, you ARE his mother. You really aren't worried too much about the elders and cong. (I am assuming here), you are worried about your son. Well, you hold the truth, so you have the passion for the truth. If indeed you decide to make this incident the wind beneath your wings that carries you away from the org, use that passion on him.
Isn't it crazy beyond belief that you hold a certain perverse "fear" of the elders? Of losing your son?
I made it clear what I was finding out to my mom as I left. (not all of it, just the blood issue, which was the "passionate" issue for me at the time). She could not argue with my reason, and I did not indicate at any point that shunning me would be an option I would tolerate.
I was of course bluffing somewhat, about that last. Truth is, I was scared shitless that she would take the company position and consequently all the rest of my family would follow her lead. What can you do if they choose that course? I played a bit on her built in guilt, letting her know that, in a way, it had always felt like love from my parents was conditional (very true, but she never mapped it out, or intended it to be that way).
Your son of course, being a snot-nosed JW male godlike one, may not yet have any guilt about love being conditional. If he has a child, it may be more likely to have crossed his mind.
Of course, the dynamics are reversed in your case, but it is your son, you have the truth, and shunning is NOT an acceptable human option, in spite of 75 years or so of WT crap advising it.