I would have thought that as you hadn't actually adopted him, he wouldn't be classed as your child? Just a previous child of your wife?
If that is the case then it's not yours, you shouldn't have to pay, however, it is up to your morals /conscience if you class the child as yours, and you acting as it's Daddy? Maybe put yourself in the kids shoes and decide?
Haha... Now you are testing me. I haven't got a clue. But I would have thought that the Courts take more of an individual circumstances approach to it and look at the case and make an order based on that. In this case there is only one dad and he is a step dad. So it would seem more likely. But where there is a biological father with parental responsibility paying CM then maybe not so much.
I know there is a thing called 'loco parentis' in law, but not sure how it works tbh.
Simple: because the child is a person with feelings and he would feel hurt when he sees that his half brothers are treated differently, they have things that he can't have because they have a dad with parental responsibility and he doesn't. Yet he has known the man as dad since the age of two.
The poster clearly loves the child and wants a relationship with him.
All i'm saying is if you love that child the same as your own, why differentiate them financially? I'm not talking about legal responsibilities and duties here.
If you are one of those that doesn't pay - then don't pay for any of them. But to pick and choose seems cruel to the child. That's all.