Thanks so much for the additional information and clarification.
First, let me say how sorry I am about your grandfather; even though he passed several months ago, I'm sure you still miss him each and every day.
It's quite possible that the stress involved in his death triggered a condition which Buster was able to mask. Unfortunately, this is all too common a phenomenon when it comes to a cat.
I can't know for sure, of course, but when older cats start to behave in odd ways, then several possibilities come to mind:
1. Hypertension which is rarely primary in a cat but rather usually secondary to an underlying disease such as Diabetes, kidney issues or hyperthyroid disease. Routine blood work will help to determine if any of these problems are present but it's going to be a challenge, I suspect, to take his blood pressure....or at least to reasonably evaluate the results.
2. A brain mass might cause unusual behavior but will be difficult to diagnose without an MRI...something I suspect you wouldn't put him through and I don't blame you.
3. Dementia or Alzheimer's can cause older cats to manifest unusual behavior but usually there's other signs present such as confusion or staring off into space or loss of housebreaking.
If you can get him to the vet's office, I would warn them that he's been unpredictable of late. He may surprise you, though, and be a perfect gentleman but if he's not, this won't be the first cat that your vet has seen who acts out at the vet's office and it won't be the last!
They should be prepared to handle him with gloves, if necessary, and to take it slow with him so as not to agitate him any more than they have to.
I wouldn't allow this concern to keep you from taking him to the vet; he may be a challenge but trust me when I say that they'll should be prepared to handle him. I might call ahead and explain your concern to them; if there's any hesitation about having him seen at this particular practice, then I'd call around and find another one who feels comfortable dealing with a cat who might have the potential to lash out.
I'd also ask to be taken into an exam room as soon as you arrive so that he doesn't have to sit in the waiting room where the presence of barking dogs or other animals may make him more anxious. Or try to schedule the first appointment of the day.
I hope this helps and that he behaves himself (which actually wouldn't surprise me at all if he did!) . Deb