Petal has more or less taken the words out of my mouth - again !
The current set up seems ridiculous - and unworkable long term. The pair of you have to look at what's in the total pot, and redistribute that as fairly as possible .... what worries me (and no doubt it does you) is that you say your DP has suddenly announced that maintenance to his ex has "doubled" from what they agreed - just like that. What does that mean exactly ? ..... maintenance doesn't just double arbitrarily ..... it's either set by a court order, set by the CSA, or else your DP has agreed to pay her double. If it's the 3rd option, you and he need to have a very serious talk very soon because that's completely unfair. He's in no position to agree any rise in maintenance without discussing it with you first. So far as his son is concerned, he's legally obliged (unless there's an overriding court order in place) to pay what the CSA's calculator (on their site) suggest ..... if he can afford to pay more and you both agree that, then fair enough, but you are a couple and you need to be fully involved in that decision. Forgive me if I'm speaking out of turn, but in light of everything else you've said about him booking holidays for him & SS and presenting them as a fait accompli, him spending all his money on SS, him insisting on SS holidays no matter what, no effort made to address the drop in income caused by you losing your job and so on, it did cross my mind whether perhaps he might even be trying to protect "his" money (i.e. the money for SS) by saying maintenance had doubled, in an effort to effectively force you into contract work (which it doesn't sound like you want to do) ?? Either way - you need to get to the bottom of what's going on. You said that all DP's money goes on SS ...... but if that consists of flights and maintenance, then how will he be able to double the maintenance anyway ? Sorry for all the questions but it sounds a bit suss to me and I hate to suggest this but I'm worried that you've been taken advantage of up until now, and what will happen if you can't continue to sub him like you have been doing.
Thing is ...... and apologies for being the voice of doom, but what would he do if he didn't have you ? He couldn't possibly spend all his money on SS and still afford to live himself. How would he eat, get to work, where would he live ? There is something very wrong if he's literally not contributing anything to the family pot, and he must be either paying ex way over the odds, and/or spending "his" money on other things you don't know about. As for this proposed holiday ..... really, if he's so insistent upon it, he should pay for it himself. And if he can't, well then, he can't. If you are at the stage - or fear you might soon be at the stage - of choosing between flights and shoes, then there's no way that any money should be frittered away now.
Sorry to say all this LAW but I can't help feeling that he's not being straight with you somewhere along the line. For the sake of your (soon to be) 3 children you need to find out what's what and insist that "his" money is allocated fairly. Am still gobsmacked if he's saying all his money goes on SS - and of course if ex is demanding more because she's pregnant, that's absolutely nothing to do with your DP. He can NOT continue supporting (and some by the sounds of it) just one of his children (if indeed that's where all his money's going) and leave you to support the other three all on your own - that's crazy. If he's so blinkered/selfish/scared of ex stopping contact or a combination of all three, that he refuses to look at this situation objectively then despite the fact it would be terribly scary, heartbreaking and very hard work, I'd be seriously thinking about whether or not I wanted to stay with this man. Why should the lives of four people (you and your 3 children) be dragged down because one child "must" be maintained at a particular level no matter what ? As I said before, it's your generosity enabling him to be so selfish and narrow minded - if he didn't have you he'd soon have to get more objective about this.