You have my sincere sympathy & understanding. I'm in the same boat as Flipper. My daughter & SIL are pioneering in China. It took me 2 years to track them down. Sent her an email giving her my contact info and told her my heart & door would always be open. I got the expected canned reply, not referred to as 'mom' and told her loyalty was to Jehovah and that HIS heart and door are always open.
It hurts, but I realize it took me until age 45 to wake up & 48 to actually chuck the whole thing. So I have hope that she will eventually wake up. It may take a few years but by reaching out I have assured her that no matter what, I have not, nor ever will turn my back on her.
By trying to reach out to your son, you are in essence saying, son my door will always be open to you. When the time comes that he wakes up to TTATT he will know that you will be there for him.
Just hang in there, you are not alone in this situation. Leslie