I'm fairly sure he's just mucking you around, but to a point I can understand where he's coming from.
He's already covering Sunday, Monday school runs and after school, Tuesday school runs and after school every week to facilitate OP working.
OP is now saying that she also wants him to cover Friday and Saturday EOW in order to facilitate her social life.
So if he was doing Friday, Saturday, Sunday, then Monday and Tuesday evenings one week followed by Monday and Tuesday evenings the next week, then he might as well have them overnight on Mondays and Tuesdays which would make it 50/50, formalise that and not have to pay OP child maintenance. Without the overnights he's still having to pay the CM but to all intents and purposes he's having them 50/50. If he's doing the school pick ups and after school care then them sleeping at OPs is really neither here nor there in practical terms even though it matters for CM
The whole set up, both the existing arrangement and OPs proposal is based around what days work for her to facilitate her work and her social life. I suspect that's pissed him off.
I suspect that he doesn't really want 50/50 and wouldn't do it and that's just a threat. But at the same time, he's already working to a timetable set up to suit OP's working hours and it's OK for him not to want to change what he's currently doing in order to facilitate OPs social life.