Associate Professor sounds good to me, especially if his appointment is temporary.
You could consider Co-opted Professor.
Interested in Language
I'm trying to translate a title of a Japanese person to English.
A literal translation would be "Specially Appointed Professor" (of a university), although I know there is no such thing in English-speaking countries.
It's different from an emeritus professor, assistant professor, associate professor, etc.
It seems he was appointed as a professor by the university exclusively for a project.
Is there a better way to say it?