Honorifics: -kyouju vs. -hakase

I am a graduate student at Notre Dame and I have a Japanese collaborator from RIKEN coming to my office tomorrow afternoon to ask some questions about a computer code used in my field a lot. I wish to be polite in addressing the man and was looking up honorifics other than sensei since the wikipedia article made it seem like that was not suitable for people who are very accomplished academically.

After some searching I found -hakase and -kyouju. I have seen a lot of mention of hakase but not much of kyouju and was wondering what the difference was and which was better.

If the man from RIKEN has a Ph.D. degree, you can call him "++ hakase" or "++ sensei". "++ sensei" is more common and sound natural in JP.
If the man is a technical staff or not an academic staff who has non Ph.D. holder, call him "++ san".

Kyouju(full professor) is an academic position in teaching schools for tertiary education. RIKEN doesn't call their staff Kyouju. RIKEN doen't have a Kyouju position. The principal researcher or higher position is equivalent to Kyouju position in RIKEN structure, so it depends on the man's position.

But there are many Kyouju staffs who have non Ph.D. degree, so it depends the man is an academic staff or technical staff you should call him.

Thank you very much for the information!

The man holds a Ph.D. and an appointment with The Center for Nuclear Study at University of Tokyo as an assistant professor.

Since Kyouju is for full professors, I will use -hakase unless he corrects me.

Thank you again!

Ask him what he wants to be called when you exchange each others business cards "meishi".
He may hold the Ph.D but he is still an assistant prof. (jyo-kyoju, not kyoju).
By asking you can feel if he is a humble man or his ego gets in the way.
Perhaps sensei is most acceptable, if you want to show him your respect and put him on the pedestal. Otherwise ending in "san" is good enough.

If he is an assistant professor, I would call him "++ sensei". It's because he is an academic staff at teaching institution.
I don't think he is familiar to be called "++ hakase" if he is a Japanese citizen.
I would call a person "++ hakase" who has a Ph.D. but serving a PD or research associate position.

I don't think anyone call a Ph.D holder as "hakase" at universities in Japan. that is too ridiculous, because almost all persons have Ph.D degree.
even if he is a full professor, students always call him as xxx-sensei.
thus, xxx-sensei is almighty.
even if he is a postodoc., you should call him as xxx-sensei.

if you know the reality of academic labs, full and associate professors are funding machines (generally, not doing research), and assistant professors, (research associates,) and postdocs (and graduate students) are real researchers.

PD is not a teaching position, is a research position under supervised by a professor.
Those lab members call PD as "++ sensei" but PD is a half researcher and student.

Ask him what he wants to be called when you exchange each others business cards "meishi".
......
By asking you can feel if he is a humble man or his ego gets in the way.

This is the most ridiculous thing I've ever heard.