I'm sorry to hear that. How long before he graduates? Is there still time to reason with his mom?
I had to address this issue from many angles with my husband over our son's last year of high school before he was open to the idea of it. It was almost too late to start the application process for acceptance after graduation.
If there's time maybe she can open up to the idea of respecting his choice. As Cappytan stated, be there for him, he may need your support.
Sometimes it becomes more about the parent and how they feel it makes them look as a JW then what the child really wants. I had to keep emphasizing to my husband that it was our son's life and he had to live it as well as needed to be in a position to support a family without relying on us. I also said things like, everyone is not pioneer material.
I had to talk with my son privately and let him know that, no matter what anyone else has to say, that i respect and support his decision.
We even had family tell us that we made the wrong decision, and that Jehovah's holy spirit wouldnt be with him.
It just breaks my heart, the control this religion has people's personal lives.