There isn't one. What you are thinking of were commercial promos for his (at the time) upcoming album. He was never featured in an episode.
The episode that had the commercial breaks was the Season finale,"Peter's Progress"
Season 5|Episode 4 Saving Private Brian
PTV
Thanksgiving
He's never appeared on an episode of Family Guy. He did however appear in commercials w/ Stewie during Peter's Progress
"Patriot Games" Season 4: Episode 20
The Splendid Source
No Meals on Wheels
Mother Tucker
Model Misbehavior
PTV
Thanksgiving
The episode was : The Tan Aquatic With Steve Zissou. Season 5, episode 11
New Kidney in Town
Emission Impossible & Yug Ylimaf
Fox-y Lady
Season 4|Episode 3 Blind Ambition
Season 7|Episode 8 Family Gay
Internal Affairs S10|E23