How do i remind my dad after 3 yrs he owes me �600!?

my dad is going to india to see some family he hasnt seen in a few years, but i paid for his last trip and he still owes me that money! thing is i borrowed it from elsewhere and now this person wants it back...how to i 'gentley' remind him that he needs to give it to me back be it in installments or lump sum