Should I tell my father he has been diagnosed with Dementia?

I would make excuses as to why I'd want/need to go with him that doesn't revolve around his inabilities. My Mom was bothered by me insisting I go to doc appointments with her. But I told her as her DPoA it is better I'm always "up to date" on her meds and treatments, since it is often a lot to keep track of. At 93, she lives in the house next to me and still does some very limited driving. I preemptively get groceries and items for her so she has no real reason to go out on the road. But she gets bored and uses shopping as entertainment. I gave her an iPad with games on it that is simple enough for her to use, and ask her to do "favors" for me around my house and yard (cutting veggies, watering plants, etc). She would never accept that she is starting to have cognitive decline so there's no point in "going there".

Focus on what your Dad is still able and willing to do, even if you have to make up the activity (like folding "laundry", pairing socks, sorting nuts and bolts, cutting old clothes into rags, etc). "Purposeful" activity is the best for the mental health of seniors, if at all possible.

Dementia is not the reason why your father shouldn't go to the store alone. The reasons are the difficulties he has which are caused by the dementia. It may seem like semantics but actually the difference is very important.

What symptoms are his particular dementia causing? For example: does he have difficulty finding his way to, from or around familiar places? Does he sometimes forget what he is doing in the middle of doing it? Does he become anxious and confused when there are multiple conversations happening, or a lot of noise? How's his mobility - his balance, stamina and so on?

Depending on what his issues are, frame your advice in those terms. For example: "it's a long walk from the car park to that store and it'll be really busy just now. I'm going this afternoon, won't you wait and come with me?"

In terms of discussing and explaining dementia and how it affects him, that should already have been done by his doctor. Has it?

I’d think carefully about what he ‘shouldn’t do’. If there are risks, how to minimise them. Could you take him to the store, go and have a coffee while he shops by himself, then go back just before the check-out so that you can see if he has anything crazy in the trolley? That would work in many mall supermarkets, and make him feel more independent. I’d guess that you can apply that logic to other things you have doubts about. If it works for you (yes, it’s time consuming) it could make you both happier, perhaps. Keep the ‘no’ for really difficult situations – you don’t need excuses for the minor ones.

I was always taught to be honest & I get wanting to be upfront.

But what would explaining the D diagnosis accomplish?

Will it help change his behaviour?
Would he believe you?
Would he think you were lying? Would this confuse him, anger him, upset him?
Would he understand Dementia is a life-limiting progressive condition that will steal his independence?
Do you want him to? 😥