Hi all. After a bit of advice. Ex and me split 2 years ago. DS is 1.5yo. I changed jobs 3 months ago and the agreement re contact ex and I have is that he has DS on a Saturday (whilst I'm working) the length of the contact depends on which shift I am on how early I start/late I finish etc and at first relied on my mum stepping in with childcare at time ex couldn't do (eg 7am start at work) which she was more than happy to do.
Ex has recently told me he wants ds for the whole time I'm at work, longer hours and I was fine with that and did warn him that I sometimes have early starts or late finishes but would be able to tell him which shift I was a week in advance. He agreed and all was fine.
Until this week, when I informed ex that my shift would include dropping ds off to him at 7:45am and picking him up at 4:30pm. He then went back on what he'd previously said and told me 'I can't do that' 
no other explanation suggestions or alternatives.
AIBU to think he can't just pick and choose and what would you do/should I do now? Contact is not court ordered we've done it all ourselves and up until now has been pretty good. Thanks in advance kind people!