Your grandson is old enough to get a divorcement from his parents if he feels he is being abused. The Son and daughter-in law are just plain mean to mistreat your grandson.
My son who lived with his Dad at 16 refused to attend any more meetings. He was 6 foot talk and his Dad and he got to a fight, but my son was able to stand up for himself and quit meetings. He has never attended anymore and it has been almost a year now. My youngest son has moved in with his 21 year old brother and has much more freedom. So both my sons have left the JW. For your grandson's sake talk to the school he goes too. You might be able to talk to him there by phone to see if he wants to pursue living with you. I knew of another your JW girl who fought to live with her grandmother. The girls Dad was outraged but stopped contacting the girl. They consider her apostate now. The Grandmother and her mothers sister have helped her get away from the JW's.
These kids need all the help they can get, and usually schools are supportive of helping kids get out of cults. They helped my son get the support he needed.
write me if you want though the messaging they have here and we can talk.
Ruth