Even if it weren't, it didn't sound dignified to me and it should’ve been. It is the National Anthem. And why couldn’t he have worn a barong? =)

Anyway, RA 8491 “An Act Prescribing The Code Of The National Flag, Anthem, Motto, Coat-Of-Arms And Other Heraldic Items And Devices Of The Philippines” states -

Sec. 37: “The rendition of the National Anthem, whether played or sung, shall be in accordance with the musical arrangement and composition of Julian Felipe.”

Sec. 20. The observance of the flag ceremony in official or civic gatherings shall be simple and dignified and shall include the playing or singing of the anthem in its original Filipino lyrics and march tempo.

Read Patriotism 101 (Do’s and Don’t’s re the National Anthem and the Philippine Flag)