I'm glad your Opa has you there with him though this stage.
As in e.g. 365 * 128? Or 365 * 8? The latter is realistic, the former is a bit unbelievable. Even the latter is a violation of the "magic 7"[0].
[0]https://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus...
I admit to not taking emotions into consideration here.
When you're multiplying large numbers you only need around 5 or 6 items.
His talent was mainly that he could factor super quickly, and had the times tables memorized up to some absurd level. So that just cut a lot of thinking out of the task.
I am unsure about chunking for random large numbers though. Mnemonic champions chunk very large numbers (I don't have sources on hand), but normal people generally don't.
The original multiplicands are honestly the hardest to remember. Don't materialize all of the intermediate results, and you can stay within working memory. You'll need to chunk a little, but mostly only 2 digit numbers.
Personally, I use fingers to keep track of the 10s place, and work from left to right. Less overwriting that way, and you start with an underapproximation that you refine gradually, which means you can stop early when estimating.
My Opa is also the most hardcore motherfucker I've ever met. It's not that he didn't exercise. He still heaves around 80 lb bags of sand, he's building a goddamn cottage almost by himself, he's still almost immune to pain. He's the toughest man I know, and even on the days he has trouble with fairly basic spatial tasks he can multiply three digit numbers in his head faster than I can write them down. His short term memory is still better than mine. He's still smarter than me. He's so stubborn he keeps falling off ladders every week because despite how much he has lost he just refuses. to. give. up.
I can't understate how fucking scary it all is. The man is unstoppable when he puts his mind to something. Time has not worn down his will even a tiny bit. He breaks bones more often now, but he shakes it off, does the PT, and it's like it never happened. But even that isn't enough when age decides to take you. If your spine wilts, all the exercise and work in the world will not save you. When you can't sleep more than two hours at a time, what do you do? You can't force yourself to sleep. You just slowly go nuts, a little more each day.
Aging is the real process of senescence, not just of getting older. Dementia, weakness, and frailty all seem increasingly like symptoms of a larger bodily/immune system degeneration. The inability to exercise, the constant pain, the constant sleep deprivation- those things could give dementia to a young man too. You can stave off the bodily decay until you start aging, but not after. Some lucky few, the Jeanne Calments[1], seem to be basically immune to senescence. They follow the path you'd expect, of a body slowly wearing out. The rest of us eventually hit a point where our bodies suddenly start to break down, and things like exercise just no longer produce the same changes they used to. The systems that handle growth, repair, immune responses- they just stop.
I'm sorry if this is bleak, but it hurts to see him like this. I just spent a couple months helping him put up siding. He wants to finish that cottage so much. I hope it's not his swan song.
[1]: https://en.wikipedia.org/wiki/Jeanne_Calment