The actor also starred in the 2007 film Mo and lent his voice to a character in the animated film Arthur and the Invisibles with Freddie Highmore, Mia Farrow, and Madonna.

Sullivan also starred in a fair few movies while working on Malcolm in the Middle, including Joe Dirt with David Spade and Dennis Miller, Unfaithful alongside Richard Gere and Diane Lane, as well as voicing Sheldon the seahorse in Pixar's Finding Nemo (via IMDb).

Since Twelve, there's been little to no updates on Sullivan. He's relatively hard to find on social media, and there are no verified accounts to his name. There is an Instagram account with a substantial amount of followers, but there's no way to tell whether it's actually legitimate. The profile does have more recent photos of the actor though, who has grown up considerably since he was last seen on screen. 

While Sullivan has stepped away from the spotlight for now, it's clear that love for his performance as Dewey on Malcolm in the Middle lives on.