I would just buy the tickets soon. Xmas holiday travel is always tight.
You can always use DL miles later for something else, at 1 cpm with PayWithMiles.
If you can, transfer Amex to DL - instantaneous.
If you do not have Amex, just buy the award ticket ASAP and if need be buy his ticket alone with cash.
Or book his ticket on AA and book your flight on DL to add to your miles.
or if your date is cheaper on DL, book your trip on DL at a lower price in miles and buy his ticket later with AA/UA or cash.