Sounds like son probably knows step mother to be already, so chances are they are living together. If this is the case then is it such a big deal for the child? It’s a one day event presumably?
When the time is right am sure dad will tell child. Why do people always think these things will have such a a negative effect? Child may be excited and happy for the big day! Unless of course you want to plant some negative seeds in his mind?
As for the ‘right to know’, my step son got into his dads car aged three clearly primed from mum to say ‘mummy’s got a baby in her belly’, that’s how we found out he was getting a sibling. Step son survived the new sibling from the new partner. He also survived the wedding that we didn’t know about a few years later, to the dad of said sibling.
Of course half the mums on here will prob feel completely differently (double standards anyone?) about their rights to get pregnant, married etc and not inform ex husband...
it’s really not such a big deal. If he is usually a good, caring parent then let him be and stop trying to decide how he should live his life with ‘your’ child. You split up because you were different, accept this and he will live his life his way, your ‘shared’ child will be fine. I’m sure he won’t be trying to tell you how to live.
Your Marriage to him is over, you can’t carry on controlling him anymore, even using your child as a reason wont work. Maybe you are jealous he is moving on. Be happy that your son will have a stable life with his settled dad. Or would you prefer a stream of women?