My childs father picks and chooses when he wants him: Advice?

My heart is racing due to anger. So my ex-MIL was cut off a few months ago due to very toxic behavior towards me and negatively affecting my 5yr old. So his father lives with her. She last saw him in January when my son was at his dad’s. After that, my son refuses to go over there. i leave him; he old enough to decide for himself. Obviously there is a reason he does not want to go, which I don’t know as he’s not saying. Just says he doesn’t want to go. Maybe he feels the lack of interest from her side. Ps she is always welcome at our house but refuses to visit here) So now this weekend, she has all her children over with her wife and boyfriend, and all of a sudden, they insist my son come over. His dad promised to come and pick him up today. Didn’t pitch. And now I receive a msg saying I need to have him ready tomorrow for him to fetch him. Wtf you think my child is a toy? You can switch on and off to give him attention when it suits your family. I am so mad because I’m the one who always needs to make up stories as to why they don’t come over or why they don’t keep their promises. I’m dont, fed up my child deserves better.