First question, do you have to sign off on them? Are you responsible for checking figures or do you just have to enter them and he is responsible for the outcome when he signs and the supervisor responsible when approved?
Figure it was an honest mistake, but still bring it to his attention (How did you get this figure Mr. X? Using
www.oanda.com , I came up with this figure. Where did I go wrong?). Then ask where he got his currency conversion rates from. Say that you just want to double-check his math so that he doesn't short-change himself. In my old company, if it was paid for via AmEx, they used those exchange rates for the entire trip. Here, we use oanda.com for the OOP expenses.