If he's Irish a safe bet is Danny Boy or Galway Bay - preferrably both.

I heard one Black Country duo booked to an Irish wedding. They played their hearts-out to indifference till the break and one guest asked if Aunt Siobhan could do a song - she sang a couple of notso well known standards and brought the house down.

They played all their four Irish songs at different speeds for the rest of the night - to applause.