Do you know what I would do?
Next time the dsc are due to come, I would absent myself for the three nights before.
Appreciate this might be hard if you work - but I'd really really try to do it - ask for example if you can stay with a friend, or get a cheapy Travelodge near work.
Let him know the morning of the day you 'go'. Pack your bag, and explain, perfectly brightly and politely, that you have no intention of being his whipping post, his rubbish bag for all his (perfectly understandable) feelings about his situation with his children. That you've spoken about it before, it's made no difference, so you really need to get across to him just how damaging and corrosive it is. Then say - 'I'll see you after work the evening the DSC arrive. I am no longer prepared to spend the three days before their visit being treated like a lump of shit, so I intend to remove myself from this.'
Tell him that this is the first step - next time, if it happens again, you'll be away for the three days before and the DSC visit too - he can deal with them himself. The point of that will be to demonstrate to him just how much worse things would be if you weren't there. Just how much you support him, and his DSC, despite him using you as an emotional punchbag.
And, if he then can't either sort himself out OR get help to do so, you'll end up moving towards separation.