Whatever you decide to do, you sound like a great mom! :)
Does your family (or the boy's family) have a history of people getting married at a very young age? I ask because this is common in some/many JW families. It was common in mine. If both kids seem like normal kids, then I wouldn't worry about it and let it fizzle out on its own.
If there is a history of marrying young, then I would go to the lumber store and start boarding up the doors now. And if they do ever become sexually active, he may put pressure on her to marry young. The fact that he is not baptized is a good thing, so he may not. Most JW kids get baptized between the ages of 14-18. I would wait and watch this kid. My bets are that he wants all out of the JW religion, but if they talk him into getting baptized, that changes things ... a lot! It's not like they are ever given a choice to stay or not, but if you're baptized, it's so much harder to leave.
Hells bells, woman! You have a tough decision! Good luck!