i guess that would go with his character.

i watched s1 ep 2 last night. when jon asked ned who his mom was ned told jon that he is a stark, that he shares his blood. they would talk about his mom next time they met.

im sure he did tell him at some point before that he was his son, or at least told others. i doubt he could have been that slick with words with everyone but i bet you they do a flashback to that convo.

he did tell robert the womans supposed name. i dont know if that was also some slick way to not lie and that was a name his sister went by that not many people knew.

the other thing that i questioned about ned last night was why he would allow jon to go to the wall if he promised his sister to protect him. i guess because ned really didnt believe there was an actual threat.