I'm truly thankful that your husband is on the same page with you. It will make it easier to drop the bomb on his mother about her actions and why this must stop or else the visits will stop because this can be extremely confusing for your son. It can literally lead to him getting angry with you, the parents, and have him backpacking his way to grandma and grandpa's house because they will give him whatever he wants when he wants it. For him, it is a win-win situation - as a teenager anyhow when the rebellion kicks in. I flat told my MIL just as I did other family members that when we discipline our children, they will accept their punishment and not attempt to land on the door step of another family member (ie. grandma).
My husband's oldest brother and his wife had a similar problem with their only son during his teen years. They made it extremely clear that once he decided he was grown enough to leave home, he was only 15-16 yrs old, he would NOT be living with anyone in the family. If he thought he was grown, he could get a job, find housing, and pay for utilities and housing and groceries and transportation on his own. He would NOT go live with family members expecting them to keep him up. Of course he showed up on his grandmother's doorstep (my MIL). She said it was the hardest thing to do but she told him that his parents said to tell him no and she was telling him no. She went on to tell him if he though he was grown then he could very well take care of himself.
It took 2 weeks of living in his car eating junk food like nabs and such with nobody to hire him because of his age before he came home. After he came home, they had no problems with him afterward. He tried to come here, I told him what his parents had said. He called his grandmother and grandfather in Pennsylvania at the time to try to come live with them but received the same refusal and why. He was ticked off for the most part but when he came home, his attitude changed, he realized his education was important, his chores, and learning how to be a man all had to be accomplished before he was ready to go anywhere.
I'm proud to say that today, he works in underground telecommunications as his dad and my husband did for many years until his dad went to work in Iraq and my husband was unable to work after suffering 2 heart attacks, a stroke, and a quadruple bypass, which threw his diabetes from juvenile diabetes into a quandary.
My nephew got married some 6 or so years ago. His wife is a teacher and they have a beautiful little girl. They moved to Florida where his wife's people are and where her job is at but fortunately he had work there as did my husband and BIL before he met his wife. One of the companies offered him a job in the area, which he took after meeting his future wife and the relationship became serious.
According to what I hear, they have had to put some boundaries in P. as well with regards to family and grandparents - not his parents of course because it costs too much for them to fly or drive that far to see them whenever they want but we all keep in contact via the internet and telephones. His dad contacts him when he can although usually it's an email because of where he is and communication is sketchy at best on a good day.
I do wish you the best. I hope your MIL and FIL find something else to occupy their time. After the children are out of the house, this is when parents/husbands and wives get to know each other all over again - or so I'm told. It's what my aunts and uncles have said and done. They take the occasional class at the local college in subjects they didn't take during their college years. It also allows them vacation time together as a couple.
It sounds like your MIL and FIL need a hobby, to take some classes that interest them. This is there free time: free of kids, free of parental responsibilities to a degree, and time to spend with their spouse learning all they can about the person they've sat across the table from for all these years yet were unable to do much to learn about each other due to raising children. Now is that time.
I wish you the best of luck and hope the news comes across as concise and clear and that they abide by your wishes because your husband is NOT your son's brother. He is your son's father! You are your son's mother and between the two of you, you are this child's PARENTS!
Warmest regards and all my best wishes for a favorable outcome on yoru behalf and that of your son.