I can see you're having a very hard time. It's problematic when you see the person you knew and react how you always have done and get a different reaction than expected. It's because he's now a different person to some extent.
Meanwhile, he's desperate to be the person he was, and he isn't.
So when you tell him that he knows how to go back, it triggers all sorts of things.
- Anxiety that he used to know how to do this and doesn't now.
- Annoyance that you're telling him he can, when he can't.
- Fury that you've told him how to before, but he has forgotten again,
- Anger that you're patronising him, rather than answering the specific question that he asked.
None of this is your fault. You shouldn't have to be moderating your response because he blows up. But something has obviously changed recently (since you don't say this is a chronic issue) and that's concerning.
If, post TIA, he couldn't do something like this - then it's obviously not someting that fixes easily.
But if this is recent, (and possibly anyway) then you need to get him back in front of a health professional to find out why his behaviour has changed. It could be sporadic minor TIA's or Vascular Dementia, or a fair few other things. At least any medication needs reviewing.
At the very least, like anyone that has anger-management issues, he needs someone he can talk to about what's triggering this.
I daresay he'll fight you though, because its another admission of incompetence for him.
I feel for you - it is so hard.