Backing out of my sister's wedding last minute

I posted a few hours ago about this situation. This was the post

I'm so sick of this fucker. He cares more about the hoes he fucks then his own kid. I have 3 kids and my middle child has a different dad. He was concieved when my husband, who's the father of of oldest and youngest, were broken up. He treats my middle baby like his own. My son's father couldn't give af. Why I'm so angry is because my son's father was supposed to take him with him this coming weekend because I have a wedding to go to. I'm my sister maid of honor. We are going Friday, helping set up some stuff Saturday morning. Have the wedding and our flight home will be Sunday morning. My son does not want to go because we are all staying at the farm and my middle baby, bless his heart. I love him so much. He has certain issues. He doesn't have low self esteem. He has NO self esteem. He's very shy, which is why he hasn't made friends at all and he's already in the 8th grade and he sometimes has problems at night. It's not nightly but enough to where it's embarrassing. Nobody is mean to him about it here. He just puts his bedding in a corner and me or my husbands gets his stuff and washes it. But people in a house full of people made him nervous about that and just nervous in general. And a hotel isn't an option sadly. He asked his dad back in APRIL! If he could stay with him that weekend and he said in text "Yeah son that's fine!" His dad knows about his issues. His dad has been hyping him up. Saying they're going fishing and out to eat. Blah blah blah. And he texts ME. Not even his own son. ME! And says "Sorry. Can't get Zayden this weekend. Unique wants to spend time together". Unique is whoever he's fucking now. I told him he needs to tell his son that and he left me on read. Then I told him he's a pathetic excuse for a father and he's trash who doesn't try to get better. I'm so sick of him getting his ONLY child's hopes up just to break his heart again. Like why even promise and keep hyping him up. Now I don't know what I'm gonna do. He's a POS ugly ass mother fucking excuse for a father anf I'm sorry. I know I sound so immature. I'm just so fucking pissed. I can't fucking stand this motherfucker!

Update

I've been speaking to my husband about what we are gonna do and I did tell my son his dad was getting him since his dad wouldn't. He teared up and went to his room. I know he's not going to want to go. I'm in the wedding. I'm the maid of honor and my sister is expecting to get to see her niece and nephew and BIL. She's paid for the hair and make up and my other kids have been so excited about the wedding. But I know Zayden doesn't want to go. Would I be the worst sister in the world to back out last minute? I keep typing a text to my sister and then deleting it. I just don't know any other possible solution. I don't want to do this to my sister. But my son's father has put me in a shit position.

Edit: My husband is the one driving my sister's fiance's brother to pick up their wedding cake.

Edit2: I kind of need more than just don't back out of you sister's wedding. I'm trying to find a solution.

Edit3: His dad's side are all drug addicts or insane. But the main reason he wanted to go with his dad is because his dad is the only other person outside our close family that knows he has a problem.