My husbands mom doesn't hang pictures of our son...advice?r

My husbands mom always displays the other 4 grandkids that she doesn’t get to see often cause the parents dont bring them around… yet our son hasn’t been “displayed” since he was only a few months old but she gets to see him every weekend for a few hours since his mother lives with mine and my grandmother. My grandmother watches our son while we pay bills on Saturday for a couple of hours, but when he is over there she barely comes out of her bedroom but then when the other grand kids are around she comes out to interaction/play with them. I feel lit is wrong because one day my 4 year old is going to understand that he is being treated differently than his cousins and that hurts my mom heart more than anything.