yes, i think he is prog. Prog isn't just about long songs and complicated pieces of music played university teachers. If you listen to some of his compositions, they have musicianship that not even classical composers could even fathom.
If you listen to I Just Can't Stop Loving You, there are amazing vocal harmonies, and very werid key changes, e.g. the key change at the end of the song goes from Gm to G#m, that is only one semitone. Not a well known key change, but it is very experimental.
Jackson also really believed in solo instruments as a major part of music and composition. E.g. in his live shows, he allowed his band to go into funk jams, and he even instructed them to just go with the feeling and to experiment with the music itself.
IF THAT'S NOT WHAT BEING PROG IS ABOUT, THEN THIS GENRE IS AS STEREOTYPED AS ANY OTHER GENRE.