Here's my 2 cents. Focus only on your grandchildren.
I suspect you are correct that there may be some jealousy going on with the other grandparents. That's very sad for them but it's their problem. I would not discuss any of it with your son or daughter in law.
Be the very best grandparents you can be. Do the things you love to do. Go eat those "hamburgers" and if your grandson ever says he no longer likes hamburgers, be gently with him. Tell him something like "I'm sorry to hear that because I know how much you loved them in the past. We can eat something else but if you ever want "hamburgers" again just tell me and we will go."
It very well may be that the other grandparents are telling him things but you will never know for sure and it really doesn't matter.
As long as you make the time you spend with your grandson as special as you can, he will hold onto that as he grows. If he feels safe and loved with you, he will hold onto that as he grows and as he grows he will come to realize some truths on his own.
I always find it sad when grandparents become jealous over other grandparents. They are thinking of themselves and not the child. The more love a child can have the better.
My grands live 2000 miles away and my former daughter in law is remarried to a wonderful man. His parents have accepted the kids as their grandchildren and I couldn't be happier. My grands see them more than they see me and that's okay. They are truly loved and that's all that matters.
