The American Pie actor, 31, is currently on location shooting Fox's The Good Guys, staring Colin Hanks and Bradley Whitford.
Klein is set to guest star on the police comedy's season finale.
"He's doing fantastic," says a close friend. "He loves comedy and these guys are all fantastic."
The actor's road to recovery has been challenging but, the friend tells PEOPLE, "being on set again with such a talented and welcoming cast has done wonders for him."
Klein is also in talks to reprise his standout role as 'Oz' in American Pie 4.