If you are still considered a witness they will have to listen to you if you say he's not mature enough to make the decision & that you have prayed about it
If they don't you need to bring out the big guns with your wife & threaten to ban him from meetings until his is 18, in any legal action she wouldn't stand a chance - the legal system & social services don't like cults
Ask his mother why she is setting him up for her to have to shun him at some point because it will happen, kids these days will find out the truth at some point in their lives
Does he appreciate that the WTS frowns on people marrying too young, fearing they may not be mature enough to enter into a lifelong relationship, which is to be taken very seriously??
Thats a laugh tho', I have never known a bunch of people who get married so dang redneck young (we all know why).
Most other groups would never allow it fullstop not just moan about it, its why witnesses have such high divorce rates considering their values