How to ask student for more information in order to grant makeup final
Unless this is a very urgent trip, then there's no way I'd allow the change.  There are always students who want to play "airfare wars."  They buy airline tickets for a date half-way through finals, knowing full well they might have an exam scheduled late.  They think that coming to me and showing me their ticket, "But I already spent $480 on tickets!"  will leverage compliance from me.  The best they'll get from me for their manipulation is that I'll give them an incomplete and give them their final MUCH later (so they have to retain their fresh knowledge a couple months.)  

If his sister is getting married or the Army Reserves has called him up or is mother is hit by a bus, then OK, I'm happy to accommodate.  But I didn't take "But I have plans" from my own children as an excuse.

I'll mention here that my syllabus contains a sentence that says plainly that I won't budge just because you already bought your ticket.

To your specific question, I'd say to, or e-mail the student and say something like, "Unless this is a crisis, then I can't move the date of your exam."  Then it's up to him to convince you that it's a crisis.