You are in a tough spot.
No. The simpler solution is to lay ground rules for your parents when they look after him. He is your child not theirs, if you say no cult propaganda then no cult propaganda it is...That wouldn't work at all. Even if you clearly stated your position to your parents, they would continue to secretly indoctrinate your son.
I am not going to tell you what you should do. I can only say what I would do were I to have children and find myself in that situation. I would not take chances that my son or daughter would be successfully recruited. If necessary, I would find an alternative to my JW family watching my child. If further necessary, I would say whatever was necessary to be clear on how I felt about my child being told such bull crap, even if it were to cause me to lose contact with my own parents. It's my child's future well being at stake.