"I cut my full time work to 3 days a week to accommodate helping out ... I work 29 hours over 3 days to enable me to have Thursday and Friday off ... I suspect he will not allow us access now."
Let's be blunt. Your son is a git.
It is one thing to make such big adjustments to your working life to meet their NEEDS, quite another to make such effort just because that's what they WANT.
Your son prioritises shopping and meeting friends over caring for his own daughter. And over your need to work, to care for your mother, to have a life you can call your own. And suspecting he won't allow access suggests to me he must have hinted about this already
.
Call his bluff.
You've made quite enough accommodations, it's time to tell him to take it or leave it. You said ^"They can't afford extra childcare which is why we agreed to help^" - you need to really understand that. They need you, far more than you need this being pissed around. It's time for your son and his partner to grow the fuck up and act like parents, not children. Their child, their responsibility. You were willing to give the help that you could, but if they're going to be greedy selfish pigs, they can swivel.
Be plain. 'This is what I will so, take it or leave it.' Some people see other people's niceness as an invitation to be messed around; it would seem your son is one such person. Stop inviting him to give you a kicking. Push back, establish boundaries, and tell him where to get off. Don't fear a withdrawal of access, he really can't afford to actually do that.