D.B.
I'm with you. I don't see the point of meeting "a brother" when your son barely knows "a father." Your son barely knows one of his parents, and now your ex is planning to make that more complicated by introducing two other strangers, a 3 year old and a stepmother? Your son is crying because he doesn't understand his father, and just as he starts to have a superficial relationship by phone, your ex wants to change that or complicate it.
I'd put my foot down, I think. Your ex has no set schedule, so he's not "entitled" to anything. Maybe he thinks your son will help him bond with the little one, or maybe he thinks the little one will help him find something to do with your son. Perhaps he thinks kids 3 and 8 will naturally have something in common? I don't know. I think you should consider some way of saying your ex can talk to your son if it's based on getting to know your son and having the call or visit be ALL about your son. Say that you want him to have a relationship with his father and you welcome it, but it's not going well by introducing the idea of other people. I'm wondering if your son feels a bit abandoned by his father, and now there's a new son who shares Daddy - in fact, he has the biggest share. Tell your ex that your son needs to feel he's special in Daddy's eyes and he needs to establish a relationship of trust and bonding before he's ready to meet anyone else. Dad should focus on your child and his life and interests; Dad should not be requiring your son to focus on Dad's other family.
Your son is a child and he doesn't have a say about bedtime or going to the doctor or wearing a seatbelt. He absolutely DOES have a say about being in uncomfortable situations with a dad he barely knows. I can't imagine sending him several states away to live in a strange house with strangers, and I can't imagine Instant Family showing up at your door either.
The fact that your ex gets angry is a big red flag. And you're not the bad guy to your son if you put the brakes on this. You're just a bad guy to your ex - and who cares? Maybe you can say you would agree to X number of phone calls (are you doing FaceTime so they can see each other, or is Dad this disembodied voice over the phone line only?). You can agree to Y number of visits in your state so your son can get totally comfortable over time. Then, he will maybe want to meet Dad's family - but the father-son relationship has to be paramount. The trick is getting your ex to agree to it and see the wisdom of it, and that it will make things better in the long run for everyone. If your ex would agree o joint sessions with a child psychologist about how to make this work, you can say you'd be up for it. But it sounds like he's digging in his heels.
I have a friend whose son has anxiety and other problems. He's actually hospitalized now for mental health issues. The father moved several states away and remarried (no kids) and he rarely sees the son and daughter he had with my friend. He occasionally talks by phone with his son in the mental health hospital, and he has said some inappropriate things, to the point that he is not allowed any unsupervised phone calls or visits at all. My friend would rather her ex be an involved and caring father, but it's not happening. And yes, he blames her for not letting him coparent, yet he bails on "his" vacation weeks, weekends, and holidays, and he has no idea what's going on in school or the hospital even though he's on all the forms, is allowed to call the school, and can access the website. He's pretty useless, so while my friend says nothing bad in front of the kids, she's not bending over backwards to give extra opportunities. She gets everything in writing from her ex - texts, emails, etc. - and she saves them in case she needs them. If he calls her, she notes the date and subject matter. You might do that.