I agree with Lin. If someone forgets what has happened then it is unkind to remind them time after time. I know some people argue that people should be told the truth but if that truth causes untold distress time and time again then why would you persist? At the end of the day, this is about finding some approach that helps your dad and with dementia the best thing you can do is find solutions that are the least stressful and makes the person as content and happy as they can be. It is clear that you care a lot for your dad, so, perhaps the time has come to practice the little white lies that the rest of us have had to use.
As to your dad's dementia being worse since your mum died. This is very often the case. It does not however necessarily mean there has been a deterioration it could be that you are now seeing the extent of the shielding that your mum did when she was acting as his carer. They will have had their routines and she will have become adept at helping her husband out in the many areas where he was struggling due to the challenges of the disease. Once you take the partner out of the equation, the person has lost their prop, their support system, so the extent of the dementia becomes all the more visible. It is also a testament to the caring that there was between them and why he constantly asks where she has gone.
So sad but so good that he has you to look out for him and to take over that support that he relies on.
Fiona