Absolutely as hellsbells said.
It's a classic bluff, as soon as I saw this my first Q would be 'how much does he do with the kids now'
There are plenty of involved dads who would quite rightly fight for 50/50 for all the right reasons.
Then there are the emotional abusers who use this line to blackmail and frighten you. Without fail, they all 'struggle' having the kids to themselves currently, prioritise their careers, don't pull their weight with the kids and never have.
So you call the bluff and yes watch him back pedal like an Olympic back-pedaller taking gold!
'Yes, about your suggestion of 50/50, on paper this is a good idea and I will be interested to talk it through and see how it could work. That will mean that I can work more and develop my career which will obviously put me in a stronger position. Also I presume we will both in the future have new partners - it would be much fairer for both of us to have equal free time to develop our new lives as well as equal time with the children, sharing the responsibility.'
I would put money on that being the last you hear of 50/50 as the sneers of 'you think I'm playing babysitter so you can go out shagging?' start up instead.
Result: you will have residency :)