When I quit the WT & divorced my boys were about the same age and not yet baptized. I had a sit down with them and talked to them about waiting as long as possible and used Jesus as an example, he was 30, yet the bible shows he was very spiritual lad at age 12 when he was left behind at the temple. He was the perfect son of god, why did he wait? Answer: to get more training.
I let them know that what ever they decide, I would support them. Have a heart to heart talk with your son, make him realize that if he ever changes his mind in the future, it could cause his mother to never speak to him again, so he best be sure he really wants to make that big of a decision this young in his life.
All the best.