His son was a super strong, super-fast,x ray vision wielding alien (Thats all we know that they know about in the films) and you expect him to naturally think "Hmm, people won't have a problem with this, tell everyone what you can do Clark"?
Seriously, I think lots of people just can't put themselves in characters shoes. I mean they think they can but to actually be put in that situation, and to just think about it with none of the emotional attachments that these characters would have, or the fear they would have about their child, is totally different.
Pa Kent's role wasnt to turn Clark into Superman, it was to turn him into a GOOD man, someone who is compassionate, kind, loving and thats exactly what he did. If it wasnt for those lessons he would never have become Superman in the first place.
Also he was right about humans, if Clark had come out at any point before he did we wouldnt have been ready. Instead he came out when the only choice he had was to fight to protect the planet he loves from invading super beings. You think people would have been so trusting of him if we didnt need him to save the planet? They show in the film that only once it becomes absolutely clear that they needed him was it that they actually stopped trying to shoot him and blow him up.
The tornado scene (and this film in general) gets far too much hate in my opinion. Pa Kent showed that he was willing to give up his life to protect those he cared about. You think without that motivation Clark would have got to where he was later on?