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.

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.