It’s not about what the child wants though is it for you. Mothers like this give women a bad name. What you are actually asking is at what age do they take into account what the child wants because I don’t like what is happening so that is what my child will feel because I have decided so.
I was honestly gobsmacked reading your post.
“I found out about her via social media” (so you have gone looking online and then complained he didn’t tell you first but you did not give him chance to because you are watching his every move online, he is allowed privacy).
”I wasn’t happy about it cos I knew this would mean less time for my son” - No You made that up in your head and decided it to be true, because it’s easier to get angry about your negative feelings than it is to look for positives. Having a step parent and step siblings can often ease the load and mean more time together. She could be cooking dinner ect and allowing him more time with his son but you have decided it must mean less time before they even met. Your child will pick up on your negative attitude and it will harm his relationship with his father and step family. It’s dangerous.
“so I told ex that this new relationship better not affect my son” - you did not need to do that at all, you clearly see him having a new partner as a threat and seek to limit his time with his child because of this. That is so cruel.
She didn’t meet my son for about 5 months (sounds respectful)
then my son comes back to me and tells me they all went to the beach together (child appropriate and fun, but god forbid you would look for ways this is positive for your child 😬).
I will admit I was fuming. (That your child had been taken to the beach with his dads partner. Have a day off seriously.)
He didn’t discuss it with me first or ask if it was ok. (Firstly, he doesn’t need to, it’s not your business and he can do what he likes with his life. Secondly, I absolutely don’t blame him for not telling you based on your thoughts feelings and reactions to what you have said so far. It sounds like all you would have done is oppose it and focus on the negatives and try to control the situation which is unfair to them. He has not told you to keep the peace so it can be an enjoyable experience for his son and his partner).
I did message and call him to tell him I was not happy with this (why? To achieve what other than trying to control his relationship with his son and his partner? You might not have been kartwheeling and it might be hard but suck it up sister because other peoples happiness is not yours to destroy. There was no need).
but he ignored it. (I’m not surprised).
This carries on then after they had been together 8 months ish my son was introduced to her children. (Again a respectable time frame and child appropriate).
Again, I wasn’t consulted about it. (No need for you to have been, this is their life).
I told ex he was selfish for doing this as son is shy and wouldn’t like being around so many other children (how do you know what he likes and doesn’t like? I assume you will be homeschooling him if you don’t want him ever around more than 3 children and think he will be unable to adapt to that amount of children.
This will go one of two ways
You will either have a hard look at yourself and the work you need to do to allow you to sort your own feelings out without projecting them onto your child and your ex, and to allow you to become a better mother and coparent in that sense. If that is what you are interested in doing then I suggest coparenting podcasts. Moms of tampa is brilliant but there are loads of others.
Or you will take issue with everything he does completely ignoring that he is a brilliant dad wanting a fair amount of time with his child and wanting to treat his child respectfully and lovingly, eventually doing all you can to destroy the loving relationship they have and limit the time based on jealousy and your own feelings towards them as a couple. Your child will end up hating their dad but in the long run eventually you for destroying their happiness.
you may not have wanted a blended family for your precious baby but you got one. It’s time to grow up now.