The kid is 14. He's articulate. He should be able to testify on his own behalf.
First of all, he can say how the religious environment of the JWs tries to pit him against his mother in favour of the JWs/his father. That, in legal terminology, is called "alienation of affection". It's like a kid being around a father who refers to the child's mother as being a 'slut' or a 'whore', in an attempt to coerce the child to distance himself from the mother.
Secondly, he can explain how the religion impedes his ability to have friends Outside the Truth™. The old School brochure, the one we had to bring in on the first day of class every year, to explain to the teachers and headmaster why we couldn't do all the fun stuff that other students got to do - celebrating birthdays, Christmas, Hallowe'en, participating in debate club, sports or other extracurricular activities, does a really good job of that. I seem to recall how some of the Caleb and Sophia videos show how kids are expected to Make Disciples™ of their classmates rather than establish true friendships, as well as how it's okay to tell a "friend" that their same sex parents are living a sinful life that Jehovah hates.
Third, there's that 1994(?) Awake magazine with the cover that has all those pictures of now-dead JW kids who died because they were brainwashed into refusing blood transfusions.
Let the Watchtower's own literature be used against them. Let the kid say that he doesn't like being told he's going to die if he doesn't follow the religion. Let him say that he just wants to be a normal kid, and to be able to make up his own mind when he's a grown up. Let him say that Jesus didn't get baptized until he was 30, why do they want him to do it now, when he's not even old enough to drive a car, or open a bank account, or enter into a contract?