Okay ..... if SS is abroad I understand CSA might not apply (though - and I might be talking out of my hat here, I did think that some countries had kind of reciprocal arrangements for maintenance which were based on CSA guidelines). Anyway ... there is still nothing stopping DP paying a CSA rate based on his salary. He can go on their site, use their calculator (which will also take into account the other 3, well, 1 at the moment) and send his ex that amount. SHE does NOT get to decide what the maintenance should be .... that's a matter for him and you to decide, so long as it's minimum CSA. The ex may not be happy but so what - if they were still together she'd have to manage with the "woeful" amount he earns. Why should it be any different because they're split and he's had the good fortune to meet someone better paid than him ? .... and what, realistically can she do about it ? DP's conscience should be clear because he'll be paying a sum as per legal minimum required in the country where HE lives. Glad to hear the flat is going ..... no wonder he spends all his money, but what a bloody waste when he doesn't even use it. All that money could have gone into savings which'd be helping you out now.
Think, in your current circumstances, he is also going to have to re-think contact. Flights every month without fail are fine if you can afford it but not when you can't. He's being utterly ridiculous and irresponsible by refusing to talk about this .... even if you did spend all your savings on this, they're not limitless, and there's now a big question over your future income.
As I said before, if he refuses to address this by having an adult conversation, I'd refuse to fund his holiday. Whatever it took to make him take responsibility and work out with you how both of you are going to survive financially and how contact can be afforded - which is not necessarily the same thing as contact continuing on the basis it has been. And of course the issue of maintenance has to be addressed too. If he insists that the ex "must" get what she demands, and/or what he thinks is a "fair" sum (and that sum is unaffordable) then he's not just placing your children at a financial disadvantage but also, at an emotional one because he is, effectively, saying that SS is "more important". That may not matter to the younger children now but it will in the future and could be extremely damaging when they work out how favoured their older brother was.
Quite honestly, it doesn't sound as if you'd be any worse off - at least financially (appreciate it's not quite that simple) - if you and he were to split. In fact you could be considerably better off - assuming you can find similar work again - as he won't be scrounging your savings, and, if you went to the CSA, he would have to pay for your 3 children.