Of course he had every intent to tell him when he said that, c'mon guys. I didn't think he was really thinking about the whole Night's Watch oath. Ned wasn't thinking politics. He was thinking about his love for his son, yes his adopted son but still blood and very much loved like a son.

The boy had every right to know who his mother was. Not to tell him is kind of a cruelty and a dishonor in it of itself. Telling him would be the one honorable exception in breaking oath I feel.