It's not unusual for me to encounter a situation in which a parent has gone to a particular doctor for years, even decades, who is now hesitant to accept that
there's a dementia process going on. Social skills often are the last things to go, so if the parent seems normal during a visit, the doctor won't notice anything wrong. Sometimes, too, the parent and doctor have become very close over time -- they may even be the same age -- so a lot of dynamics are going on that can cloud the issue.
If you've obtained your dad's consent to discuss his medical files, ask to schedule a 10- or 15-minute discussion with his doctor. You could say something like, "It may be difficult for you to see what I'm seeing because sometimes Dad seems on top of things, but here's what else I'm noticing that worries me."
If the doctor keeps resisting, look into getting a second opinion. A full-scale evaluation of dementia symptoms can help to determine whether that's really what you're seeing or whether it's a thyroid problem, depression, something else, or a combination of things.
Depending on how the second opinion goes, you can ask the original doctor what he thinks about the assessment. If he's still unresponsive, switching doctors is probably in your father's best interest, if you can get him to agree to that.
Think about seeing a geriatric specialist. Just as parents often take their children to a pediatrician, it can be advantageous to have your father see someone who specializes in the medical concerns of people his age.
It's really hard to assume a parental role with your own parents, especially with issues like dementia. At some point, though, you just have to accept that roles change and summon up the courage to take a more decisive lead in your parent's care, stepping into areas you wouldn't have before.