Yeah, what Jinx said. Luke didn't have much of a Force presence before he starting learning the ways of the Jedi. Also, how was Ben ever going to have that conversation with Luke?

Luke: "So how did you REALLY know my dad?"
Ben: "Oh, I taught him just like I'm teaching you. Then he turned to the Dark Side so I WATCHED HIM BURN TO DEATH BESIDE A RIVER OF MOLTEN ROCK. Also I cut off his legs and one of his arms. 'Cept he wasn't really did. Also I saw your mom's vajayjay."