{{{Teppy}}} I can't imagine how hard it is to watch that happen.
It's been happening gradually; he frequently doesn't understand why [anything] outside of his routine is occurring. And that includes a can of tuna, or apples.
The good thing is that with persistent reminders, he can still create a new routine -- he's robustly physically healthy, but as a 78-year-old man, he has started to need to take meds to avoid a urinary tract (and/or prostate, maybe?) problem from recurring. At first he didn't remember to take his meds, because he had never needed to take meds. And he didn't remember what they were for. And when the boys reminded him of a really bad urinary tract issue that was the triggering incident for the medication being prescribed, his initial response was that he was fine now, so he didn't need drugs.
But the boys persisted in calling to remind him to take the meds, and told him if he wanted to avoid ever having a catheter again, he needed to take the meds every day. Coupled with a day-of-the-week pill caddy, he's improved his record of pill-taking to every day, though he does miss a day occasionally.
So if he really wants to add tuna to his diet, we just need to remind him (1) that he wanted to start eating fish because his doctor recommended it, and (2) there's tuna on the counter and he should have it for lunch.
But really, he's going to need to move soon. I think we'll make it through the holidays, which will be important emotionally to him, but he can't continue to live alone.