Does he own such a thing as a mobile phone? With a calendar? That he can set for important dates with reminders and beeps and so on?
You could always snoop his pho -
Oh for goodness sake, just say. Not knowing is worse than knowing.
If he then goes I know, ta daah, you'll feel relieved.
But if he then does oh shit I forgot, either verbally or in body language, just be open and honest that you're upset and hurt about that.
Then have a conversation with him that was very well expressed earlier about how you do the mental load, how it's always been the day before his mum's, how he's able to remember work engagements, appointments, meetups with his friends etc and that in future
you expect the same consideration that he gives them, and you give him.
Either way you'll feel better than this.
You've given the impression that he's a good bloke and that if he's forgotten, it's unusual.
Stop the 'guess what's wrong' and don't do the tit for tat thing.