OK, have Marty, after he arrives back in 1985, dress up in his radiation suit and walk into his parent's bedroom one night with the same voice synthesizer he used in 1955.

"Vader": It's me again. Good ol' Darth Vader from Vulcan
George: Yeah, yeah, you're the guy from Star Wars who was Luke's son after all, and formerly known as Anakin Skywalker. But you look different in person.
"Vader": No, I am your son. I visited you in 1955 via a time machine.
George: No, this can't be...that's impossible...Wait, I remember one night, I was visited by...
"Vader": That was me, Darth Vader from the planet...
*Marty does vulcan symbol*
"Vader": Vulcan!.
*Marty then takes off his radiation mask, revealing his identity*
George: Nooooooooooooooooooo!

Work something out from there.