I'm inclined to state that Richard didn't know until at least the end of the movie...I say this because remember when Clark spies on them in their house we hear their conversation where Richard asks Lois about Superman. Lois becomes flustered and explains the article title meant nothing and that his Uncle Perry came up with it. It's clear that Richard is a little intimidated by Superman's return and that Lois hasn't revealed the extent of her true feelings for him. Lois would not have remembered her time with Superman in the Fortress, at leas not right away so there wasn't anything to tell Richard when Jason was born.
Much like Clark I like to think that Jason's powers developed over the five years...and then suddenly he gets a burst of Super strength (maybe like when Clark lifts the truck over Jonathon's head in Superman The Movie, I like to think this is the first demonstration of Clark's powers). Richard probably strongly suspects that Lois and Superman had some kind of relationship before he came along near the end of the movie.