As far as I know, if you have bought it using your amazon account you can't send it to his without doing all the stuff people do to share books by email (I think it involves lots of formatting and using special email addresses - see the kindle book club thread).
Might be easier to contact amazon and say you bought it in error, get them to refund, and then send your father a gift voucher and get him to buy it himself.
I think it's really about time Amazon made it possible to give other people kindle books, but they don't seem to have got on to it yet.