Because I am really starting to reach the end of my tether with him constantly chopping and changing DS's overnight as he pleases. 
Our arrangement is that he has DS wednesday tea time (this used to be an overnight but was stopped by ex), and saturday two p.m (ish) to Sunday noon (so an overnight). DS is autistic and doesn't like change, refers to his calendar and his daily picture tiemtable at various points during day and basically likes to know where he stands, what's going on, the plan, etc.
He is constantly changing these around to suit him usually at last minute although sometimes he will give me about 4 or 5 saturdays as a list in advance that he can't do. 
Today he dropped DS off, waited until he had got home and then text - "you haven't forgotten I can't have DS saturday or even friday next week so I am having him sunday night instead have you?" I usually just remain agreeable and fix another night for DS, usually the friday when he says he can't make the saturday.
He did not mention next week to me. He is claiming he gave it to me in a list months ago with other dates. I know he didn't as I wrote them all on calendar and also one of his nights out next weekend was only arranged a few weeeks ago so he can't have done (he doesnt know I know when this particualr night out was planned).
He basically argued with me and me with him, things got a bit strained, he begrudgingly apologised (follwed by a but...) and he has his own way yet again.
Incidentally I had a very very rare night out planned next saturday which I am now unable to go on. Friend will be pissed off with me.
How much of this chopping and changing do I have to put up with???? I had been keeping a log of these incidents but have been lax with it for the last month.