I had a similar experience with my grandmother. Her motivation and curiosity left her. I tried to interest her in new things a number of times, thinking she'd be so much happier. The data, though, led me to decide she was in fact happier when I just accepted who she had become.

This is my greatest fear, more than death: to stop learning, to stop creating, to stagnate. I wonder if nutrition or exercise has an impact on this. I would eat broccoli every day if I thought it would improve my chances.

Check out the whole-food, plant based diet: https://nutritionfacts.org/video/alzheimers-and-atherosclero...

https://nutritionfacts.org/video/the-alzheimers-gene-control...

Credentials of research scientist behind the whole-food, plant based diet: https://wfpb-wolf.netlify.app/maturity.html

OTOH,the brain needs animal fat for DHA:

https://www.psychologytoday.com/us/blog/diagnosis-diet/20190...

I'm vegan and supplement DHA via this product: https://ca.freshfield.life/products/vegan-omega-3

Grab a joint/vaporize some loose leaf herb; I don't know why people don't give cannabis to their disoriented, chronically disengaged elders more.

Because they are already disoriented and disengaged so they don't need more.

On the contrary, in many cases with medical problems.

Hmm, maybe try figuring out what things he likes and/or can engage in without effort - whatever he can do well without thinking.

And maybe track this over time. Borrowing the analogy of HDD/SSD corruption, which can impact critical filesystem metadata just as easily as file contents, it's impossible to predict how that fundamental capacity to engage will shift and change in confounding and lateral ways.