I wonder if you'll answer my questions or just skirt around them.
Of course I'll answer your questions - as you asked so nicely 
You keep spouting about Courts, family law,legislation etc. Why? Wtf has that got to do with a scenario such as this where this man appears to want to take the role as 'dad' to his stepchild, and his stepchild obviously wants to spend time with him and likely views him as a father figure?
This man has also decided to have another baby with the OP. He has therefore chosen to change the responsibilities he has to fulfil. Some of which are required by law, hence my reference to legislation and the differences that are legally perceived between stepparents and parents. It appears that the Law is out of step with the views on this thread - after all, if it is so damaging for the DC in this case to have his stepdad removed from his life, then it must be damaging for other DC's to. Yet, the law does not protect them.
If the OP's relationship breaks down - should her DC's support from his/her father take account of his fathers choice to maintain as relationship with an unrelated child? Would it be OK for the OP's DP to say to the OP - "I can't afford more than the legal minimum in CM, because I'm supporting a child whose biological father has abandoned him?".
Why should the OP's baby get "less" (irrepsective of whether the OP and her DP are together) because his/her Dad was previously a stepdad to other DC's?
It is generally considered unjust that a NRP's Child Maintenance payment is adjusted to take account of stepDCs' which live in the NRP home - especially if they are in receipt of CM from their own biological NR parent; how is this different?
If he 'officially adopted' this child, would you accept that he should be treated equally to his two siblings? Or do you still have the belief that ONLY the biological children should come first?
Of course not - legal adoption is quite diffferent. Its not the "biology" per se as the legal responsiblity that is the key factor in my opinion. If the OP's DP had legally adopted his dSC, then he would be an equal parent, with equal responsiblities. But that is not the case. The OP's DP has voluntarily taken on repsonsibility for this child, which is admirable, but it should not imo be at the expense of his legal responsiblities.
If the OP and her DC will be expected to make sacrifices because her DP has chosen to continue to provide voluntary support to a DC who has two parents who have a legal responsiblity to do so, then her DP is not the "marvellous man" that he is being made out to be.