It is important indeed to address people correctly, more important that initials, one may pay much more attention to the correctness of the name and family name which that person has written in his web pages, letters and papers. Even, if you know some people who know him (e.g. colleague, students, etc.) simply ask them how it is better to call that person.
In my opinion, the best way to address a person and write his name in letters is to
copy and paste the exact thing he has written in his web page.
If the person has included his initials in his web page such as "Dr.", "Priv.-Doz. Dr." or "Prof. Dr." then it seems that those initials are important for him and should be included; if there is no sign of those initials; then it is better just to use Dr. or Professor.
If you use something and he feels it should be corrected, then he will use the correct form of his name in his reply email. After that, use his preferred way to write his name.