A little more help from the OP wouldn’t hurt. What are his (and your) blind spots with respect to movies? If he’s seen practically all the SF movies and also likes comedy, I figure he’s already seen Galaxy Quest and Spaceballs.
He’s 20 years old, so there’s probably a lot of great 20th century movies he’s never seen. Has he seen the classic Bogart movies (Casablanca, Maltese Falcon)? How about the Marx Brothers (Duck Soup, Monkey Business)? Has he seen A Hard Day’s Night? It’s still cool after all these years. How about the Mel Brooks classics like Blazing Saddles, Young Frankenstein, and the already-mentioned The Producers? Robert Redford and Paul Newman in Butch and Sundance and The Sting?
Etcetera.