How to address an academic with a PhD in Spanish? A friend of mine is defending his PhD thesis in the Netherlands this week. 

It is a custom to formally address them directly after the defense ceremony, where we use the Dutch term "welzeeredelgeleerde heer", which is something like 'very honourable well-learned gentleman' in Dutch and it very precisely corresponds to the title of PhD. 

However, this friend was born and raised in Mexico, and I would like to also address him in Spanish. I have asked several native Spanish speakers (both inside and outside of academia), but nobody seems to know the correct translation of this terminology.

Does anyone happen to know this trivia?
I am a mexican, I dont think we use anything other than:


  "Doctor" + Lastname


or if you want to use "very honourable well-learned gentleman"


  Honorable y muy sabio caballero