Pretty standard call here I think. He could be drawing to hearts or have tptk. He didnt make too many hands ott apart from specifically 78, which is unlikely given pre flop action. I doubt he plays a set this way otf either.
Just looked at the spoiler, so my advice pretty much stands, he could easily have jammed the turn here with 99 if a 7/8/J came ott rather than a 9.