To not be ready for the OW to meet my kids despite her being pregnant with their step sibling

As others have said, what an absolute shit your STBXH is. I am truly sorry you’re going to through it. Children aside, just bear in mind that he can do to her exactly what they’ve done to you but you won’t be the one dealing with the fall out. My ex DP did exactly the same thing to me and it was horrendous but karma has its way of coming back around eventually. No woman deserves an unfaithful man.

If I were you I would not try and block them from meeting OW and I would be actively trying to help him emotionally deal with new sibling because unfortunately it’s happening and he’s going to need a huge amount of support and coping mechanisms to deal with it. Be open to talking to your son about it and paint it as a lovely thing for him to have another new sibling just as your own baby is for him. I’m sure you can get books for autistic DC aimed at an age appropriate level for him to help. I know none of this is your responsibility and it will take an incredible amount of strength and courage from you in order to do this. Your ex is such a frightful wanker but if your child can see you’re at least outwardly amicable about it then it may subconsciously help his anxieties he may have about it.

I wouldn’t push for any extra contact between your ex and your son. Whilst things are raw, very much deal with contact on the basis that your child is happy with that your ex is offering. Even if it’s paltry it will give him a small amount of exposure which will give him more time to adjust. Your DC have one amazing parent in you who has their best interests at heart and that is all they need. Take your strength and comfort from them.

Reading your update. Honestly op you know your dc and works best for them. Your poor dc.

So your ex left you pregnant for another woman who he then promptly got pregnant as well?

What a colossal tosser.

What a prize your ex is. Unbelievable. Horrible selfish people the pair of them.
But there's not really going to be a right time. If you wait, he has a stepmum and a baby to adjust to. If you don't wait, he has stepmum and then a baby to adjust to. It's a lose lose situation. Poor kid.

You and you ex split cause of another woman not the childrens fault dont punish them for his actions children are not weapons to be used as n when suits you neither are they novelty items to be rolled out for show they are people with thoughts and feelings

In my head I would have to separate the whole thing from my life with my son and young baby to stay sane and I would think that my ex husband has to manage his own life and new decisions and how they impact on my son, however hard this is to do. If your son doesn't enjoy being around his dad and new partner, and eventually new baby, the visits will get shorter. I would just make my son feel extra special while he was with me as he will be dealing with so much. Sorry you are going through this.

From your update, I would wait until there is an agreed way forward tbh. Your@ eldest DC needs more stability than an NT child.

Why should they ever meet. They have nothing to do with you.

Yanbu. What a horrible man. However as there's a baby on the way the sooner the better so your 7 year old has more time to adjust to this new situation.

Awful. I’d be fighting the urge to take the kids and disappear. But I’m not advising that, really. It’s just how I would feel.

Yanbu but as one of your children’s parents is a shit one, you might not have much choice.

A seven year old is not going to understand the awful timing,and if he is having contact with his father then there will be baby talk and stuff going on and someone is bound to say something to him. I think it will be better if it comes from you. Try to put a positive spin on it about how exciting it will be for him to have two baby siblings and how he is going to be such a special big brother to both of them. I know it is an awful task, but it’s about his feelings and expectations, not about yours atm.

I am only sorry you are having to deal with the shit your ex has piled up on your doorstep.

Does your DS like story boards?
My own DC does better with picture boards to prepare for change he's not use to. You could collect pictures of things babies might need to get him invested in knowing change is about to occur? (Or better yet his dad could)

No advice OP but I'm sorry your STBXH is such a colossal shit.

I think I'd be inclined to be as low key about it all as possible around your 7 year old, safe in the knowledge that eventually both your children will be able to work out for themselves exactly why their parents split up.

I think biology has taken control here. I rather feel you have to strap yourself in for a rocky year and get the introduction proposed, expected and done so the baby can be proposed, expected and done. He deserves to meet his brother as soon as possible after the birth.

But, perhaps discuss with the senco or his teacher about how they think he will take the life change.

And perhaps make your house routines rock solid so he knows exactly what he is coming home to.

Gosh, OP. What a horrible situation.. No advice, but I'm sorry.

It's gonna happen either way isn't it? You can really stop it from happening if dad insists.

I'd do all you can to lessen the impact on your DC by easing the transition. 2 new siblings is going to be difficult. Even for a neurotypical child. Let them question things, be as honest as possible (I know how much this must sting, regardless). Make sure YOU have someone you can talk to about your feelings on this. Your ex is a massive shit for doing this. Obviously not really thinking about the DC he already has but it's done now. It's about damage control.

how involved is the dad? That would be my guide. I have DC with autism who would struggle massively and I would protect them from this shit show if needed. Really would depend for me how much the dad is involved. or did he just dump the lot of you?

This change is happening whether you like it or not. The timing will always be awful for your seven year old.