Hi C.,
We so want to protect our kids from things like this. I have seen it in my husbands family. His maternal grandmother played favorites to the point of excluding him from fun times with the other grandkids. He is now 35 and had very little to do with her. She only lives about 5 miles from us, but we only see her once every couple years, and we never talk on the phone with her.
His paternal grandmother however treats all her grandkids the same, even if they are her grandkids by marriage. And now that he is an adult he make sure to see her every holiday, her birthday, and usually at least once a month just to make sure she is well. He also calls to check on her and if she doesn't here from him when she thinks she should have, she calls to check on him.
I wish I had some great advise for you too, but I have noticed that you can't really change people like that. It's just how they are going to be. And yes, eventually your son will notice, but as long as he feels loved and is secure in his home with his parents and those that are a part of his daily life, I don't think it will affect him long term.
I know if he were my child I would want to "protect" him from feeling rejection from them and not send him to them either. Maybe as he gets older and can speak a little better for himself he will tell you he just doesn't care to be with them alone. Sometimes even family personalities just don't mesh.
Lots of love to your baby boy!
One other thought, I do remember my mother didn't seem to accept my neice who wasn't biologically his when my brother moved here several years ago. He came with a new wife, a two year old step-daughter (although as far as she is concerned he is daddy and that is the end of the story) and they had a month old baby girl of his own. She attatched really fast to the baby, but would always make comments when the older wasn't around about how she didn't enjoy her. And though she didn't see it, we noticed when she talked to the oldest, it wasn't with the sweet talk of a grandmother, it was always short and too the point. My brother and I both confronted my mom about this at different times and she said it is just different with step grandkids. We told her it didn't have to be but that it was her choice and the little girl was going to grow up knowing that she was not accepted. Although my mom would never admit she had the problem, she did begin to make a stronger effort toward the oldest, and now 4 years later she loves spending time with both of them and treats them both the same. I think she may have even forgotten that the older isn't biologically related. So maybe it will take the kids father, since it is his mother/grandmother, to say something. Has he stepped in to confront them at all? Maybe there is hope.