My sister had to deal with this and my Mom was highly offended that she (our Mom) would not be allowed to take grandson to meetings. She and my sister butted heads over this and it actually caused a huge rift.
So, you will need to decide how much and how strong in the Truth your parents are and how strongly they feel it is for your son to receive Jehovah's food. They will feel it is critical for him to receive Jehovahs food and will tell your son you are nutz, crazy, wrong etc., for not allowing him to go to meetings and out in Field Service. They will also tell your son not to tell you they took him. My mom tried with with my nephew and like I said, it created a 25 year rift.
This is something to really think about. I would simply not let your son stay over. I would try to visit and have them visit. If they will. With your husband DFd, they may not and will feel even more need to snag your son.
You have my full sympathy. Good luck.