My Dad's neurologist never told him he has dementia and told us not to tell him....when things get worse, should I tell him?

It is your decision, not the neurologist's, what to tell your dad.

Some people do benefit from knowing their diagnosis. My husband did. Some people have no need to know and they'd rather not. That's my mother. So, without knowing your dad I have basis for giving advice. If you neurologist had some knowledge of and relationship with your dad when he or she gave that advice, consider it carefully. If it was just generic advice about the doctor's general experience, then you are probably in a better position that the doctor to make a decision.

But, if you are going to tell, why would you wait until "things get worse"?

Has your Dad asked what the doctor said? Has he mentioned being forgetful? I don't see what telling him would accomplish from what I am reading here. It may upset him and surely, he isn't likely to retain the information. If Dad mentions being forgetful, laugh and say we all forget things as we grow older. There is nothing your Dad can do to change things so personally, I'd let it go.