The actor - who plays Steve Stifler in the franchise - joked that Biggs shows his "ding dong" in the film, which reunites the characters from the original 1999 teen comedy.
"Mum goes, 'Is that really his penis?'," he told Jimmy Fallon. "That was so weird for me, firstly to hear her say 'penis'. I've eaten dog poo in a movie, I've been peed on, but for some reason this was the weirdest thing, for my mum to see his penis."
Fallon jokingly asked: "So... Jason Biggs? Jason Average?" Scott answered: "Erm, he's a good size. This is so weird! What am I supposed to say?"
Tara Reid, Eddie Kaye Thomas, Thomas Ian Nicholas, Chris Klein, Alyson Hannigan, Mena Suvari, Shannon Elizabeth, Jennifer Coolidge and Eugene Levy will all reprise their roles in American Reunion.
American Reunion arrives in US cinemas on April 6, with a UK release on May 2.
> Seann William Scott's Goon gets new trailer, poster
Watch the trailer for American Reunion below: