OK Here’s a big what-if question…
Howard Dean is a medical doctor. IF he was elected President, would he be addressed as Mr. President or Dr. President?
Mr. President is the official title and would be the correct address.
Other presidents have been doctors, too. Not medical doctors, but Wilson, for instance, had an earned doctorate, and quite a few modern presidents have been awarded honorary doctorates. In any case, “Mr. President” was their official title.
If you’re addressing him as the POTUS, then you call him “Mr. President”. If he has agreed to check out that weird pain in your elbow while he’s the POTUS, you can call him “Dr. Dean.” If you’re a personal friend, you’d address him as “Howard,” at least in less-than-ultra formal situations.