Monday April 9, 2012 - 2.20 'Ha 'elele' Meaning Abandoned 10PM (Alex does have a small role in this episode)
Monday April 16, 2012 - 2.07 'Ka Iwi Kapu' Meaning Sacred Bones (R) 10PM
Saturday April 21, 2012 - 1.05 - 'Ko 'olauloa' Referring to North Shore of O'ahu (R) 9PM
Monday April 30, 2012 - 2.21 -'Pa Make Loa' Meaning Touch of Death 10PM (First half of Crossover with NCIS: LA. Alex will not be in this episode. Just wrapped filming over the weekend)
Tuesday May 1, 2012 - 3.21 - NCIS: LA Crossover on NCIS: LA - 'Touch of Death' 9PM (Alex will not be in this episode also)
Monday May 7, 2012 - 2.22 - 'Ua Hopu' Meaning Caught 10PM (Alex returns. Filming is this week. Wo Fat will be featured in this episode)
Monday May 14, 2012 - 2.23 - 'Ua Hala' Meaning Death in the Family 10PM (Season 2 Finale)
Peter Lenkov twitted this earlier that he is off to Hawaii for the filming of Episode 22.
Reading at the airport. HI, see u soon twitpic.com/94gfu4
— Peter Lenkov (@PLenkov) April 2, 2012
Sources: CBS, Alex O'Loughlin Rocks & Peter Lenkov on Twitter