My chemo brain was / is nothing like that. I forget stuff to an incredible degree (though I've had fun starting a new planner journal with lots of motivational stickers!) and my teens are good at reminding me. But it's more 'when stuff is' or 'what I promised to bring' or things that aren't a part of our regular calendar. Not stuff like taking the TV remote in the car (though I suppose I could do that by mistake, big as my bag is).

Who's in charge of his meds? Some of them can mess with memory, and that's worth checking. Also some of them mess with moods, and if his memory and his mood go weird, it's worth keeping a close eye on the meds. Check the side effects and see if any of them are adding up with each other.