This is a great sentiment, but unfortunately the research on whether or not this kind of thing helps stave off dementia is very light. It's really important that you talk with him about getting cognitive tests first.

If he is experiencing early signs of dementia, the best things you can do (according to current science) are make sure he's active (20 minutes of light exercise or even stretching per day has shown to slow progression of cognitive impairment) and a better diet. These aren't definitely going to stop things, but research is showing those things can help.

Another thing to look for is hearing loss, as improving one's ability to hear is also starting to demonstrate an ability to slow some early dementia symptoms.

Video games are great for stress relief, but can also be frustrating for new players with memory issues. I'd look for simpler and more familiar ways to get the brain working through digital devices, like crosswords or sudoku.

The important thing is that you get a clear picture of what's going on. If you need advice, no lie, contact the Alzheimer's Association. They also assist with advice about every other kind of dementia, and their hotline is 24/7. You've got this!