OP I see where you're coming from. Quite frankly exh is clearly not a nice or decent person - I understand changing jobs, and I understand maybe not being able to afford full maintenance for one month. But he could and should have given you more notice to plan ahead. He should have also offered you some maintenance, even if not the full amount. Even if it's only 50quid, If he's really struggling. Although to be quite honest I can't see why an adult can't save a little bit in anticipation for this - ok he may not have known for long but even saving 300 the past few months. Shaving money off his food bill. Giving no notice, offering no money at all, and avoiding your texts, just shows what kind of a person he is. So you're not being unreasonable in that respect. Of course children still need feeding.
I think the reason some posters have mentioned money is thay you mentioned struggling to get to appointments, but people see 600pounds and everything else goes out the window. For 3 children that's 200 a month. Which is a really decent amount but hardly extortionate. Especially as your ds has additional needs and the extra costs that come with that. It's not just for everyday stuff (I'm sure some months you spend more than 200 on each child and some months less) but can be put aside for treats, trips, days out, presents etc.
So exh is a child in an adult s body who can't face issues head on and has no concept of compromise. That said, if the money isn't there, it isn't there. 100percent he should have at least offered a reduced amount. But you can't do that with bills or even petrol or train costs, really. There's no leeway with companies. You just get cut off without services. O know some people are mentioning double wages, but it doesn't always happen like that. New starters have joined my team recently. For one, her last pay day from her previous work was 15th February. This company pays on last working day, so 29th March. That's 5 weeks between pay. She's lucky in that she could apply for a mid month advance. If she'd. Joined a day later she couldn't have. If she'd joined a week later she'd only have a reduced payment this month due to the cut off date. There can also be issues with the new job paying on time, or getting emergency taxed etc.
People are saying maintenance isn't optional, which it's not. However you can part pay the RP and hope that your hopefully good relationship (if you've been mature) and the fact that you've paid in on time every other month, means your xp is willing to compromise. You can't do that with bills. If for eg we all took the hard line and insisted he pay 600 regardless, he may be left without heating or electricity. No means to wash his work clothes, or even himself. No means to get to and from work and limited means to get food. Which could potentially lose him his job (issues getting in, hygiene and dress code issues) and make him unemployed. Therefore making maintenance even less. People have also suggested he get into debt to pay the maintenance, which I can't believe. Most reputable sources such as banks, m and s, Tesco etc, want a reason such as house deposit or buying a car, to loan money. His only option might be payday loans. Which is not a good idea. Again, because if he racks up payday loans his financisl situation worse and again impacts maintenance.
OP you aren't being unreasonable in that he should have given you notice. Although I'm unsure what holidays and unwillingness to ask family members, has to do with anything. Why should they step in for their son/brother/cousin who is unable to organise himself properly? And I imagine it would be humiliating to ask. You're right in that he can't give what he hasn't got, he can't. But he has acted badly. The lack of notice wasn't great. And he could have offered some maintenance, even if it was only 100pounds, this month.
Perhaps look into going through CMS. All I know is it's fairly unusual to be given 600pm on cms. It's a decent amount and again CMS go off your exps incomings etcx and there is a minimum amount. Up to you though
And sorry for the bad responses you've had on this thread - mine hasn't been completely in the Make him pay the full amount camp, buy I don't think personal comments about your finances are fair