3 Year Old Son has hated his grandma (My MIL) from birth and we don't know why and it's creating a real issue

My son is almost 3 and from the very first time he met my MIL, his grandma, from birth he has been terrified of her and has never wanted anything to do with her. Even as a baby, he screamed if she held him and cowered away from her.He's completely obsessed with my mom, but my mom makes the effort to see him at least once a week. So, at first we chalked it up to the fact that they don't interact very much. So we would bring him over to her house a lot and try to force interaction and get her him more familiar with her. It didn't work.It's very awkward when we're together because he will actively avoid her and doesn't even want to look at her. He's perfectly fine hanging out with and interacting with MIL's husband but doesn't want anything to do with her. We tried to tell her that he's just shy but that's a lie.This is what's so confusing. Why is he so terrified of his own grandma, but a repairman comes over to fix our dryer and that's his new best friend and he's showing off all morning? Or our older neighbor he stares out the window waiting for so he can stand on the porch and wave? Or the guy who mows our grass who has to stop for 20 minutes so our kid can tell him all about his book and what he did at school today? He's not afraid of people at all. He LOVES talking to people and showing off.As a result of all this, My MIL hasn't made any effort to see him at all this year. She has spent a total of 20 minutes with him in 2018. She constantly posts pictures of all her other grandkids of her Facebook but never acknowledges our son at all. We tried constantly to make plans to get together but she would always say she would have to get back to us. This Friday, my husband and I have to go to a family event and she was our only option for a babysitter. I have so much anxiety about it.Yesterday, my husband showed my son a picture of the four of them - MIL, MIL's husband, my husband, son - when he was a baby on the fridge. He said "Hey buddy, do you remember who that is?" My son's whole demeanor changed and backed away from the fridge and kept repeating "No no no." My husband took the picture off and tried to explain "This is daddy's mommy..." and my son screamed "Put the picture back!"This morning, we were getting ready for school and playing and I said "Do you want to see grandma on Friday?" and his face went stone and he said No and stopped talking to me.I know someone is going to say there was abuse, but like I said, this has happened with every interaction since birth. She has only babysat for us maybe 4 times? The first time was probably after he was 1 year old. He was already terrified of her by then.​This is so, so bizarre. Has anyone experienced something like this?​ via /r/Parenting https://ift.tt/2OiZEFM