Gassho,
It was my understanding that So Doshin had many different instructors in various branches of chinese martial arts, and so you can't really say that any one man was his instructor. This man may have taught him; i do not know, although someone else may. Also, what do you mean by Kempo? So Doshin founded Shorinji Kempo and so would not have an instructor in that style. Maybe you should make you question clearer!
Kesshu
Robert Dawson
Southampton University Shorinji Kempo Club
http://www.soton.ac.uk/~shkempo/