Have chronically ill DS (CF) with ex. Always had fairly informal custody arrangements and when we originally separated I worked full time so I had DS three days while ex had him four days. This lasted about a year, then care went 50/50.
DS then became unwell (few years ago) and ex decided to hit the bottle as a response. I took DS full time and cared for his medical needs and got ex into rehab (took bloody ages mind). It took a long time before ex was having regular access again as I needed to ensure that DS would be cared for (both normally and medically).
Ex did improve and got a job, which meant that he hasn't always been available to help with DS, but he sees him when he can. Occasionally he will go AWOL from work and us, but normally resurfaces and is responsible.
However, DS is currently going through a tough time health wise, and we've been on some intensive treatment for about a month. Ex started off being really helpful, coming to hospital when we were initially in etc. however, now DS is home and I'm doing IV antibiotics here (and running back and forth to the hosp for check up/meds/lung function/dietician) etc, Ex agreed to have DS on Wednesday for a couple of days to give me a break as I've been doing this treatment for about two weeks (and I'm pretty knackered). Ex was supposed to be working on Tuesday, then having DS on Wednesday. Except he phoned me Tuesday and was drunk, asked me to phone in work for him 
and he'd phone me on Wednesday to sort out arrangements for DS.
I heard nothing from Ex until tonight. I text him as all his phones are off to see what was happening as he didn't stick to arrangements etc, and DS wanted to speak to him.
I have been upstairs for the last three hours administering several evening treatments to DS. I have come down to an abusive message and he has basically gone off drinking again - best bit - he blames DS and the stress of his illness which has driven him back to drink.
Ex and I are normally friends but I want him to realise that he can't do this, and he absolutely can't blame DS (DS has enough to bloody cope with FFS).
Why the fuck can't he sort himself out and stick to it (I know I am being a cow, and addiction isn't like that) I'm livid on behalf of DS, and actually myself. It's not easy to deal with it all, but I'm tired of being responsible for it all, all of the time.
How am I meant to proceed now?