twin I can see that you havnt had an easy ride with you're ex, but its not you're place to say he is being irresponsible in having another child, its his business, whereas you're child together is you're business.
you're higher mortgage is nothing to do with you're ex and his maintenace money (if you had it) shouldnt go on this anyway, maintenance is for food, clothing, shoes, sweets, school trips, holdiay trips, little treats now and again for the child.
Gas & electricity would have to be paid for whether you had a child or not, you have 2 incomes coming into the house so bills should be easily paid, I manage to do it all on my own on income support.
I think a bank account for the child is a good idea, but it seem you're ex really doesnt have a pot to piss in at the moment, let alone a house where a partner comes home with another wage to help out.
Im sorry if you might feel that Im not agreeing with you but this is a message board and you will get peoples opinions whether you agree with them or not, sometimes it might even take another persons view to open you're mind and realise that what you have at home is rich compared to what other people have.