I don't believe Nicholas is the son of Jacob Riffle from Harrison County, VA. There are two different 1790 censuses out there with Jacob Riffle listed. One is at Fayette County, PA, where Nicholas here is from. However, the other census is from Harrison County, VA where the Jacob you are referencing is from. It seems more logical that the other Jacob is the father here, not the one you reference.
Now those two counties are within 80 or so miles of each other. I'm not sure how fast they traveled in 1790 through Appalachia, but something tells me not very fast. So I rule out that the same man was on both censuses. On the Virginia one, he was listed with 9 total. On the PA one, he was alone. This bothers me because I'm not sure which one arrived on the ship from Germany, the one who went to Va (now Wva) or the one who went to Western PA?