Edit the scene in Obi-Wan's home to make it so Luke knows that Vader is his father from the very start:

Luke: How did my father die?

Obi-Wan: Your father, who was a pupil of mine until he turned to evil, helped the empire hunt down and destroy the Jedi Knight...He betrayed and murdered your father, from a certain point of view. He ceased to be Anakin Skywalker and became Darth Vader. Anakin was seduced by the Dark Side of the Force...

Luke: The Force?

Obi-Wan: Well, the Force is what gives a Jedi his power... etc.

The scene continues as normal then.

Alternatively, Luke's head explodes.